When trying to change username, terminal tells me user is currently used by process
man usermod :
CAVEATS You must make certain that the named user is not executing any processes when this command is being executed if the user's numerical user ID, the user's name, or the user's home directory is being changed. usermod checks this on Linux, but only check if the user is logged in according to utmp on other architectures.
So, you need to make sure the user you're renaming is not logged in.
Also, I note you're not running this as root. Either run it as root, or run with "sudo usermod".