hello GauravG, please try to reboot your computer and then see if the update is working correctly with the full installer instead that can be obtained from https://www.mozilla.org/firefox/all/ - just download it and install it on top of your current version.

Forget about upgrading from Firefox 4.0b9 - an ancient Beta version. If the update ever completes, you may just end up with another Beta version that will want to update every week! Beta is intended for users who want to help test, and provide feedback, what will become the next Release version.

Use the full installer for the Release version, as philipp recommended, to get you on the Release version "track".

Note that if you also have Windows XP you need at least Service Pack 2 if not 3 as current Firefox will not work on Win XP SP1 or no SP.