You can't modify it yourself.
Your tier is based on your Stroke Play average. The actual numbers to be promoted from tier to tier have no been disclosed by WGT, though.
Tiers and levels are completely separate from each other for the most part, and the people you are seeing probably play a lot of games outside of Stroke Play, which will earn them a lot of experience points (to be promoted in level), but will never lower their scoring average (thus no tier promotions).