$ whoami
user1
$ su - user2
Password:
$ whoami
user2
$ exit
logout
If you want to log in as root, there's no need to specify username:
$ whoami
user1
$ su -
Password:
$ whoami
root
$ exit
logout
Generally, you can use sudo to launch a new shell as the user you want; the -u flag lets you specify the username you want:
$ whoami
user1
$ sudo -u user2 zsh
$ whoami
user2
