I'm newbie on both ruby and linux, so I'm sure this is trivial but I don't know yet. I currently have ruby 1.8.7 installed and I want to update it to ruby 1.9. How can I do that?

sudo apt-get install ruby1.9

should do the trick.

You can find what libraries are available to install by

apt-cache search <your search term>

So I just did apt-cache search ruby | grep 9 to find it.

You'll probably need to invoke the new Ruby as ruby1.9, because Ubuntu will probably default to 1.8 if you just type ruby.