3
I think that the best way of doing this would be to open the database with something like PHPmyadmin and drop all tables in the database.
Next open the XOOPS archive on your local computer, find the "install" directory in the archive, upload that directory to your existing XOOPS site, then run the install wizard to create the tables again.
Of course doing it this way any users will have to re-register. Any add on modules you have installed will still be there but you will have to reinstall them. And any themes that you have uploaded will still be available.
HTH.