Mozilla has released the build2 version of Firefox 3.5 beta for testers to get feedback. According to Mozilla this build version of Firefox 3.5 is stable for daily use. So, if you are already using Firefox 3.5 beta then you could upgrade on this build just by checking in Firefox browser help > Check for updates….. to download and install Firefox 3.5 RC1 build2 automatically, otherwise normal users can not upgrade to this build version.
So, at the moment Firefox 3.5 RC1 build2 is not available for general users and they’ve to wait up till next week to download this new build release candidate version. The build version has some bug fixes.

At the end of this month Mozilla has plan to launch Firefox 3.5 final version.