You can sign out for the user that is no longer using this box. Then one of the other users will be the active user. Check through Control Center, where one user will have the green mark.
Not sure about being a “Default User” in the sense of the total of Users and Accounts settings on that device. Verify in Settings﹥Users and Accounts﹥, and edit as necessary.
Note that apps stay registered with the Apple ID account as active when installing, even if they no longer have a registered account on the device. To remove that app-account link (as needed for app updates), delete and reinstall those apps.
Else reset and start over with a clean slate.