When I go to update it says I must sign in as an administrator. Haven’t got a clue what it means

Follow the directions in this Apple document to reset your password to one you can remember: Change or reset the password of a macOS user account - Apple Support

You are being asked to enter the user name and the password of an administrator's account on the Mac. This is required for the installation of all software, Apple's and others.

If you go to System Preferences > Users and Groups you will see a list of the user names. There may only be one. If there are multiple user accounts on the Mac some of these users may be authorised as administrators, and at least one of them must be an administrator (ie. the first one established on the Mac).

If you are the only user of your iMac the password will be the one you use to open your computer, such as after waking from sleep and after a restart - unless you have used the option in Users and Groups to log in automatically. This will bypass the password requirement, not a practice to be recommended.

Log out and then into an administrator account; unless changed, it'll be the first one created on the computer. If you don't have access to it, contact the computer's owner.

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