Actually it depends on what your Gemfile looks like. If you've specified the exact version in your Gemfile you'll need to edit it before running `bundle update rails`. This means your Gemfile might look like:

    gem 'rails', '3.2.10'

Running `bundle update rails` when you've absolutely specified the version number won't do anything. Your Gemfile.lock file (which is the file that bundler uses to determine which gems to require) won't be changed. Instead of absolutely specifying the version of Rails you want to use, considering using the '~>' specifier. My Gemfile at Airtasker looks like:

    gem 'rails', '~> 3.2.8'

That means that when I run `bundle update rails` it will update the patch version of rails. Our Gemfile.lock has the following entry for rails:

    rails (3.2.11)

By using the '~>' specifier it means that we can easily update our gems patch versions without worrying about API changes between major and minor versions.

You can read: http://gembundler.com/v1.2/gemfile.html to find out more.