How do I change my current login shell from bash to ksh under Linux or UNIX or Apple OS X operating systems?

You need to use the chsh command to change the user login shell. This determines the name of the users initial login command. A normal user may only change the login shell for her own account, the superuser (root) may change the login shell for any account. /etc/passwd file stores your user account information including login shell. /etc/shells files list all valid login shells. So first make sure ksh is installed and valid login shell:
$ grep ksh /etc/shells
Sample outputs:

/usr/bin/ksh
/bin/ksh
/bin/pdksh

To change shell to ksh, enter:
$ chsh -s /bin/ksh
You need to enter your own login password to make shell change. Now, logout and login back again to use ksh as the default shell. You can verify your shell typing the following command:
$ grep "^username" /etc/passwd
$ grep "^vivek" /etc/passwd
Sample outputs:

vivek:x:6012:6013:Vivek Gite,,,:/home/vivek:/bin/ksh

A Note About Other User

The superuser (root) may change the login shell for any account using any one of the following syntax:
$ sudo chsh -s /bin/ksh username
OR
$ su -
# chsh -s /bin/ksh username