How do I change user after login (e.g. su root)?

Easiest way is to allow direct login with the user account you need, if it is not allowed already. For accounts such as root, the direct login is typically disabled by default for security reasons. So when enabling it, have security in mind.

Particularly with SSH, you may want to keep password authentication (the most vulnerable one) disabled and use e.g. public key authentication instead. With OpenSSH server, you can do that by setting sshd_config keyword PermitRootLogin to without-password1.

= Use sudo on Login = va te faire enculer