To change a user's shell type in Linux you use the chsh command. Before using that you may want to see the curtrent default shell of the user. To see the current shell type, do the following:
$ cat /etc/passwd | grep jack
So the user jack's shell type is currently sh. To change it to bash, we'd do the following (make sure you have the appropriate privilege first):
# chsh -s /bin/sh jack
Changing shell for jack.
Happy changing shells!