Also, as above. there is aan option for various games about updates. i went through all mine yesterday after discovering it as I may be without internet for a while soon and this has to be done online (turning offline) as it can't be put into offline mode with no internet.
My suggestion is, go to your games library. Right click on one game at a time and dfo this:
1. When you right click, there will be a drop box. Go down to the bottom and click on properties.
2. The box you get, click on "updates" tab.
3. If the message in the little box says "Do not automatically update this game", change it to the other choice. "Always keep this game up to date."
What this will do is you will only have to do the next step when neccessary and anything you have on Steam will update if needed without you haveing to go through any more steps.
Now, when done, go to the top of the Steam thingie, top left corner, click on "Steam", and in the drop down, click "go offline", let it restart and you're done. This HAS to be done online. But this will put Steam in offline mode. I would assume some games may still not work like Silent Hunter 5 which needs THIER serivice to work so is a separate issue, but most games should work fine.
In future, if you want an addon, or just to update just do the same again, start Steam, got to the Steam tab and turn it back online. Fairly simple once you know where it is.