First of all, update your RVM installation by running rvm get stable.
To make sure you’re running the new RVM version, you’ll then need to run rvm reload (or just open a new terminal).
Once that’s done, you can ask RVM to list the ruby versions available to install by running rvm list known.
In the output you should now see:
# MRI Rubies
...
[ruby-]1.9.2[-p320]
...
The square brackets around the patch level indicate that this is currently RVM’s default patch level for ruby 1.9.2.
Finally, to install the new ruby version, just run rvm install 1.9.2 – and wait for it to compile!