You need to have access to the files on your server. This is usually done from your local copy on your computer with either a stand-alone ftp client or the built-in ftp client in an html editor such as DreamWeaver or GoLive etc. Many other wordprocessing and other programmes have a built-in ftp client.
You can then either modify the files directly on the server, or modify your local copy and upload it to the server.
On some hostings, particularly those offering a 'free' phpBB forum (which is free anyway) , you may not have permission to access your forum files. These 'free' forums are often offered because they carry Google ads with the Google code already embedded in the forum pages.