Using chsh one can change their login shell.

To show the shells that are available
$ chsh -l
/bin/sh
/bin/bash
/sbin/nologin
/bin/ash
/bin/bsh
/bin/ksh
/usr/bin/ksh
/usr/bin/pdksh
/bin/tcsh
/bin/csh

The following shows how the shell for oracle was changed from tcsh to ksh
$ cat /etc/passwd | grep oracle
oracle:x:501:500:oracle:/home/oracle:/bin/tcsh
$ chsh -s /bin/ksh
Changing shell for oracle.
Password:
Shell changed.
$ cat /etc/passwd | grep oracle
oracle:x:501:500:oracle:/home/oracle:/bin/ksh