How do I change an administrator's roles?

Short answer: Open the person's profile, tick or untick their roles, then click Save roles.

Steps

Open Admins and click the person's row.
In the Roles block, tick or untick the boxes you want.
Click Save roles. The Cancel button next to it returns to the previous state.

You'll know it worked when: A message confirms “Roles updated.” and the chips in the list change.

The change takes effect immediately: the person does not have to log out and back in.

Pending people too

The roles of an invitation still Pending are changed the same way. They apply from that person's first sign-in.

If saving is refused

The reason appears under the button and stays there until you change the selection. The most frequent refusal concerns the account's last active owner.

Going further

What does each role give
I cannot revoke this administrator, why
How do I remove an administrator's access