The easiest thing to do would be to delete the user and start over. If they have files and you don't want to mess with it, you can use the 'usermod' command.
The classic useradd comman goes something like this:
useradd -D -g usergroup -b /default/home/dir -s /bin/bash -f inactive_time -e expiration_date
if you want to disable the inactive_time option, use -1. If you are this user, you should also add '-G wheel' so that you'll be able to use the 'su' command. After the user has been added:
Will create them a password.