Managing admin roles is handled comparably to managing groups. Once you've entered the dialog for single or bulk users, the experience will look much the same as it did when assigning roles for a new user, with choices for User (no administrator access), Global Administrator, and Custom Administrator. The last of these will allow you to limit admin roles, though not as much as PowerShell would allow:
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Editing admin roles for single/multiple user(s)
Adding a user to an administrative role can be done in PowerShell, ...
