How do I change a logo on every product of the shop?

Short answer: Two ways: replace the image of the logo in “My logos”, and every product that carries it follows; or delete the logo, choosing the one that takes its place everywhere. Either way, finish by putting the logo changes into service in each shop concerned.

Which solution should I choose?

Solution 1 — replace the logo's image: it is the same logo, in a better file. Your real logo arrives to replace the demo's temporary logos, or you have received a version with a transparent background, or a vector file (SVG). The logo stays the same: every product that carries it follows, with nothing to choose.
Solution 2 — delete the logo and designate its replacement: two logos already exist, and one should take the other's place everywhere. For instance, the demo's “Light articles” logo and your real logo are both placed on products. The replacement must be one of your approved logos, with the same marking technique.

Your real logo arrives after the demo: there are several logos to change

The demo placed temporary logos, and replacing the image of one does not change the other ones' products:

“Light articles” on light products, “Deep articles” on deep products (named “Articles clairs” and “Articles profonds” in some demos): replace the image of each — the deep one with the deep-background version of your logo. Or, if your logo works on both backgrounds (see Light background or deep background — which should I choose), replace the image of the light one, then, once that logo is approved, delete the deep one with solution 2, choosing the light one to replace it.
“Logo pour mug”, on the mug: replace its image. Or create a mug logo from your new logo with “Create a mug logo”, at the bottom of its versions table, then, once that mug logo is approved, delete “Logo pour mug”, choosing it as the replacement.
“Broderie” (“Embroidery” in some demos), if you asked for embroidery: its file cannot be changed from its detail sheet — see How do I get an embroidery logo.

Solution 1 — replace the logo's image

On the “Shops” page, open the “My logos” tab in the left-hand panel.
Click the thumbnail of the logo: its detail sheet opens on the right.
On the image preview, click the small icon at the top left — a picture with a magnifying glass — and pick the new file, SVG or PNG. The icon has no tooltip; on a logo the demo created automatically, the sheet points to it with the sentence “Click on this icon to chose a logo file to upload”.
Click “Save”. After a few moments of processing, the message “The logo has been successfully saved.” appears.
Look at the status line above the image: it tells you whether the new file is fit for printing.

An embroidery logo has no such icon: its file cannot be changed from its detail sheet — see How do I get an embroidery logo.

What changes with a new image

The logo keeps its name and its place on the products. In every shop of your account, drafts included, the products that carry it are marked with an orange background, and the shop shows an orange banner until the logo changes are put into service.
The new file is checked again, as on upload. A fully vector SVG is approved automatically after a check, and the sheet shows “File of sufficient quality for textile printing”. A PNG switches to “File must be validated by DAGOBA before use” until we validate it. A file unfit for textile printing shows “File of insufficient quality for textile printing”. A shop already open for orders stays open meanwhile.
The print files are redone automatically from the new image: if it is fit for printing, you have nothing to send us.
The logo's versions (Standard, Junior…) are kept, and each product keeps its own. But their dimensions are recalculated from the shape of the new image: a size you had changed by hand goes back to its default value — to set it again, see How do I change the size of a logo version. With DTF printing, the price of the marking depends on the version's area, so a product's price may change.
The versions you added yourself with “Add a version” are kept too, and the products that use them keep them. With DTF printing, their width does not change and their height follows the proportions of the new image, unless a version then falls outside the marking limits: it is scaled down if its area goes beyond the largest marking area, or enlarged if its shorter side drops below 1 cm, proportions kept; their marking price may change with their area. On a mug logo or an embroidery logo, they keep their dimensions. Their print file is redone like those of the other versions.
If this logo is also a shop's header logo, the website's header switches to the new image when those changes are put into service. In a shop where it is only the header logo, no orange banner appears: the website's header changes the next time a draft of that shop is put into service — see How do I put my logo in my shop header.

Solution 2 — delete the logo and designate its replacement

In the “My logos” tab, click the thumbnail of the logo you are giving up, then the red “Delete” button on its detail sheet.
The “Delete the logo: …” window says, if it is placed on products, on how many products and shops it is used, and shows the logos that can replace it as thumbnails.
Click the thumbnail of the logo that should take its place, then “Replace and delete”.

In every shop of your account, drafts included, each product that carried the logo then carries the chosen logo, in the same placement, in the version with the same name — Standard for Standard, Junior for Junior —, failing that in the one with the same dimensions, and otherwise in the closest one in size, which the confirmation message points out. If it was a shop's header logo, the chosen logo replaces it there too. Then the logo is deleted. The changed products are marked with an orange background until the logo changes are put into service.

Which logos are offered, what to do if none is, and the details of what changes: see How do I delete a logo.

Putting the change online

Both solutions change your logos and your products in the dashboard. A shop's website, however, keeps the old logo until the logo changes are put into service: that is what redraws the product photos with the new logo. For each shop showing the orange banner:

On the “Shops” page, select the shop.
Click the orange button: its tooltip reads “Modify the shop (create a draft)”.
Click the blue button with two checkmarks: its tooltip reads “Put the logo changes into the current shop”. It only shows if you have changed nothing else in that draft. The shop's website is then updated, and the orange banner goes away once the update is finished.

If a draft of that shop already existed, that is what opens at step 2: the logo change is already in it, and it goes online when you click “Put this draft shop into service” (blue button with two checkmarks). To check how it looks on a draft before your buyers see it, see I changed my logo but the shop products do not show it.

Going further

I have received my real logo, how do I replace it — solution 1 step by step, when your final logo arrives.
How do I delete a logo — solution 2 in detail, including for a logo used nowhere.

You'll know it worked when: On the website of each shop concerned, the products carry the new logo, and the shop no longer shows an orange banner in the dashboard.