Just select the "System update" option in the "settings" tab. You can update from the internet or a storage device.
To update from a storage device you need to download the UPDATE.PUP file (Update files - http://www.ps3news.com/forums/downloads.php?do=cat&id=8). Place the UPDATE File (UPDATE.PUP) in a folder called "UPDATE" and that folder should be in a folder named "PS3" and then put the "PS3" folder in the root of your storage device and it should detect your update.
DO NOT UPDATE OFF THE INTERNET UNLESS YOU WANT 3.73... It does not give you a choice of what firmware you have to go through a memory stick (PS3/UPDATE) to force 3.55 official update and then stick 3.55 hacked on top.