Visit the Download Center for the most recent version of Investor/RT.
Setup > Preferences > Mail and Internet has a Check for Updates check box; when checked, Investor/RT will notify you at start up when a new release of the software is available and offer to download it. If you elect to turn off the automatic checking, you can check manually from time to time by choosing Check for Updates from the Investor/RT Help Menu.
If you wish to review the latest fixes and improvements prior to upgrading to a new release, you will find these on the Download Beta page. Note that you must be logged in to the site in order to download beta or release candidates.