You will need use an FTP client to connect to your website root on your host's server. You'll also need to know the FTP address, username and password to login to your site. Once you're connected, look for your forums folder (probably named "forums").. Inside this folder you will find the files. Download them to your computer, make a backup, do the modifications, then upload the modified files.
I use WS_FTP Pro but there are many free ones that work just as well.
http://www.ftpplanet.com/download.htm
If you still don't understand what we're talking about or if you can't figure out how to use the FTP client just contact your host and he'll explain everything.