Hi does any body know if I could change the data base sever that I already existing website?

Yep - you can move your whole site, files and database from one server to another.

You will need to back-up all your files and database (phpmyadmin lets you do this), (probably to your local/home/office 'puter).

You can then copy the files to the new server and overwrite a database on the new server with the contents of your back-up (again, phpmyadmin lets you do this with a sql command).

The only thing that you will need to change are the xoops_url, xoops_physical_path, xoops_trust-path (if you use protector), the database name/passworg and a couple of other bits in mainfile.php

And questions then please ask as I've done this a few times in the last few months.