How to change "root" name in a remote ubuntu 20.4

I would like to change the "root" name to a custom name in a remote ubuntu 20.4 machine. I tried the solution from Change root name in ubuntu but it doesnt work. Now I cannot log in at all.

I also checked this solution Renaming user name but the machine runs on a GUI.

What is the correct process to change the name from "root" to lets say "user1"?

Note: Please double check before marking question as "duplicate". The links mentioned are not a clear answer to the question.