From another Linux system (or the same!) or a Windows/Mac/whatever system you can access the system using an SSH client. A common one for Windows is PuTTy
If you use Linux, simply type ssh username@hostname
where username is the user you wish to connect as and hostname is the name of the server. Many distributions have disabled root logins using SSH too, so you'll need to log in as a regular user and then use the su
command to become root. When done doing root things, exit root mode by typing exit
or hitting Ctrl+D.