If you're not rooted, you should eventually get an over the air (OTA) notification from Sprint. If you don't want to wait, read the first post in this thread:
here in houston, TX been checking a few days now and seems the OTA is ready. (I already tested doing the upgrade manually from the thread here, (Your Way To Upgrade to MA6) but went back to 4.1.1 to test the OTA, which i am downloading right now.
I hear the MA6 build is not being pushed out anymore but there is a MB1 leak lurking around. Most likely the premium suite upgrade everyone is waiting for with 4.2.1.
MB1 is only 3mb.some people where reporting overheating issues with the modem, probably just bug fixes. It has been rolling out to tons of people tonight.