How can I update Ruby and Rails on MacOS 10.6?

Try using Homebrew and/or RVM

RVM allows you tO have several versions of ruby installed if you want.

Homebrew is a nice package manager for Os X.

First, don't update the built-in Ruby. Apple put it there for their use. If you want to piggy-back on it without changes, that's fine. If you make changes you run the risk of affecting apps that expect it to be at the standard Apple-installation version.

Instead, I recommend RVM for installing a separate version of Ruby for your own use. RVM makes it easy to manage and works very well. I have it on my personal laptop, my work Mac Pro, and a bunch of Linux machines.

Once you have a Ruby installed under RVM's control you can install, update and delete gems without affecting the built-in Ruby. You can install multiple versions of Ruby and of gemsets, switching back and forth quickly and easily.