Assuming you are using usermod -l username
for thisuser in user1 user2 user3; do usermod -l $thisuser; done
Or, if the users are in a file (one user per line):
for thisuser in `cat file.with.users`; do usermod -l $thisuser; done
The above 2 examples are one-liners and can be entered from the command line. If an actual script is needed (taking second example as base):
for thisuser in `cat file.with.users`
usermod -l $thisuser
Hope this helps.