Cannot access Time Machine backup

iMac Pro running Sonoma 14.2.1 went dead and couldn't be powered up. Brought it to Apple Store where techs corrected a software issue, erased memory, and installed the OS. Started it up, and as the new-computer wizard took me through the steps I failed to notice that it created a user name different from the one I had before. I was able to restore from a Time Machine backup, and those files are present and taking up space, but I can't access them. Says I don't have permission. Since I have admin privileges I tried creating an account with my old user name, but it says no, that exists. Yet I can't see a user with the old name. How can I change to the old user name? Failing that, can I wipe it clean and start over and put in my old user name and will that give me access? How's that done? Many thanks to all.