Did you ever set up an Apple I.D. for your new Mac? For Mac's running Catalina or later, I believe you can click on Apple
Logo/System Preferences/Apple I.D. and that might give you some screens that will help.
You also can create an Applie I.D. here:
appleid.apple.com.account
However, changing your Apple I.D. can cause some issues. See this link:
https://support.apple.com/en-us/HT204071
Before doing anything with your Apple I.D, you might consider calling Apple Support and discussing your update issue with the techs there. They have the capability of remotely accessing your computer that we can not do here. Click on the Get Support item in the upper right hand corner of this forum's screen. Navigate to a place where you can enter your phone number.
-- Rich