How to rename a user safely?
Safely? Don't rename at all. Instead, (make sure the user is logged out first) create a new user with the correct username, rename the old home directory to the new username and chown -R the folder to the new user.
This assumes that you're not using encrypted homes. That would require some different steps, but since I haven't done that myself, I'll leave that to someone else.
Use usermod like so:
usermod -l newname currentname