[danso]

It should be as easy as 'gem update rails' at your command line (Terminal window).

If you're just learning and creating an app for your own edification, this is not really an issue that will affect you. That is, it doesn't affect how you construct the app, so if for some reason the gem update process doesn't work, you won't be hindered from using RoR.

[bonzoesc]

This won't work. The old version won't be purged, and it'll still be referenced in the Gemfile.