When trying to change username, terminal tells me user is currently used by process

To quote man usermod :

   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".