Make sure that you're getting signal and in SprintPCS coverage area. The l;ates SW is HTC - EVO 4G (A9292) - 3.70.651.1 - 12/13/2010 .If your phone is not updating the sw via OTA(Over the Air). Please visit Sprint Service and Repair Store to have you device sw updated.
Oh gosh, why would you want to upgrade to the 3.70.651.1 release??? That was the MR release that messed up everybody's phone. In fact, there are a few threads where Sprint Admins have stated that the bugs caused by 3.70.651.1 will be fixed by the next MR release which is yet to be announced. If you have the software build prior to 3.70.651.1, I would wait till the next MR is release because this one provides more quirks and bugs on its own (just search this discussion forum and you'll see a ton of gripes about the release).