What to do with admin username on mac?

Question

macOS

Hello everybody! The problem is that I renamed the username (administrator) to mac. After I rebooted the laptop, firstly the name did not change, but I logged into the system under it. Secondly, my new old name does not work. For example, I want to change the input parameters or upload an update, I write my data, but they are not correct. Have tried many times already.
Please help)) I don't know how to be at all.