No1 do a full back-up of your current board including all your vb files and database.2 close your board 3. download 3.6 vbulletin to your PC,open 3.6 upload folder and and find install folder and remove the install.php,as you are wanting to upgrade.Now open includes folder and find config.new and rename to config.php,now open config.php with a text editor and copy the details from your original config.php that is on your server.Once you have done that FTP all files in the upload folder to your server and when asked select overwrite to all select yes to all,once done point your browser to
www.yoursite/forum/install/upgrade.php and follow the on screen instructions.
hope this helps