RubyGems can update itself to the latest version:
gem update --system
However, you always get the newest version. This may be a problem if you’re trying to replicate a known-good environment: for example, the output format of `gem list` in RubyGems 1.3.7 breaks Chef 0.8.16 in certain circumstances. 1.3.6 is fine.
Here’s how to update to a specific version—in this case, 1.3.6:
gem install -v 1.3.6 rubygems-update && \ ruby `gem env gemdir`/gems/rubygems-update-1.3.6/setup.rb
It’s also possible to revert RubyGems to an older version using this method.
(And if you’re wondering why I don’t just install the version I want, well, Ruby Enterprise Edition comes with RubyGems already. I’d prefer to update that than to install another copy.)