The typical way to update Rails is with RubyGems.  `man gem` or `gem -h` for details.

Prefix your gem command with sudo to give it administrative privileges (think of sudo as command-line UAC).

If you're just interested in dabbling with Web development technologies, PHP is the way to go as it has the lowest learning curve of the three you mentioned (Rails, PHP, ASP.NET). The downside of that learning curve is that it also has the least to offer when it comes to learning about Web development and programming.

Ruby's worth learning even if only in the sense that learning a new programming language expands your horizons.

Why the lucky stiff has written a DSL for GUI development called Shoes which you might find interesting.  It's more interesting than Rails, in my opinion - Shoes