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this. Specifically for what you're asking but for new users, you can go to Administration, Users, Add User, there you will find "Administrator" yes/no radio button. I'm not aware of any button that changes an existing user, so you can (1) figure out yourself what field to change in the database to mark them as an admin, (2) add the equivalent rights where I first mentioned, (3) delete and re-add the user as an admin, or (4) find the option somewhere else that I'm not aware of.