You've played enough ranked rounds as a Master, now you just have to get your average to 63 or lower. WGT counts your lowest (however many) rounds to determine your average at each tier, so in order to lower your average, you need to shoot rounds with a lower score than what your highest counted scores are. Confusing, yes?
For example, if WGT counted your lowest 5 rounds, and you recorded a 62, 63, 64, 65, and 66, your average would be 64. If you then shot a 65, your average would drop to 63.8 even though you recorded a round higher than your current average. The 66 is replaced by the 65 as 1 of your lowest 5 rounds.
Hopefully that makes sense, but basically you need to average 63 or lower.
Kelsey