I need to change the username from the linux (Not admin).

I changed in settings ->Users. But how to change the "home folder" name or root user in linux.

I tried using

usermod -l <newname> -d /home/<newname> -m <oldname>

But this always mentions that

username <oldname> is currently used by process <some id>

Does anything happen if I kill the process?

I logged in as root using su - and tried the same usermod command.

So, how do I change the name without killing the process?

Any leads will be appreciated. I have attached a screenshot of changed username in Settings > Users.