ubuntu how to switch to another user , how to disable ssh login

 switch to super user :

https://askubuntu.com/questions/617850/changing-from-user-to-superuser

sudo su -

switch to another user:

https://unix.stackexchange.com/questions/3568/how-to-switch-between-users-on-one-terminal

sudo  su - user2

For safety, for devop user should be disabled ssh login,

disable user or root user ssh: 

https://www.linuxshelltips.com/disable-ssh-user-login/