The usual procedure works for this. Just tap 'Help' at the top of your browser, then 'Check for Updates'.
A BUNCH of security updates are addressed in this update (some considered critical):
In addition, more stability improvements are onboard.
Update: Less than 48 hours later, Mozilla has released Firefox 3.6.8 which fixes a stability issue with some plug-ins.
The release notes can be found here and the download page for various systems and languages is now available here.