Welcome!
To update, basically just download the ZIP file from the Outpost 2 section of the website and extract it into and over your existing Outpost 2 installation. It actually will run stand-alone but overwriting it works as well.
The download includes all patches and all content that's been reviewed and demonstrated to generally work.
To play multiplayer games, you should head onto IRC. You can use the Chat link at the top of the page to get a web IRC client and join chats there. User base is kinda small so you'll be lucky if you pop on and can immediately get into a game. Generally you need to stick around for awhile and probably set up some times with other users -- most of us are working adults so we all kinda have limited time.
Happy gaming!