Help

$ gem help update

Usage: gem update GEMNAME [GEMNAME ...] [options]

  Options:
      --system                     Update the RubyGems system software

Update a target gem

$ gem update GEMNAME

Update all gems

$ gem update

Update the RubyGems gem only

$ gem update --system

From the help:

--system [VERSION]           Update the RubyGems system software

You might want to use more control with sudo or --user-install.