You must lower your average score even more. It's intentionally not made known by how much more, to deter sandbaggers. The criteria for becoming Master have recently become more strict. I've heard guesses that the threshold might today be at around 68 or slightly below. No one except WGT knows for sure.
Also, once you become a Master player, you keep that tag forever, even if your average score goes back over 80 or even 90 again.