Many solutions
:
1- as root, vipw offers you to change the entry in the password file where the last field is the shell the user will use.
2- logged as the user, chpass will help you (see man chpass study: )
3- If you want to change the shell for a group of user, you must take a look at login.conf (man login.conf study: ) where you can specify a shell for a whole class. You can change the class of a user with chpass as root.
Hope it helps you.
Fred: