Are you asking how to change your GT, or how to make the game stats show your new GT, because you can't do that.
Also, trivial information here, my GT before Crusader King 1 was
buckdeer12 (my dad made it because hes dumb sometimes and I kept it like that for months!)
Forge FTW.
3 accounts in the top 50 1v1 standard.
Good 1v1 standard non-asian Forge player
45 TS For 1v1
URL of the week:
http://wwp.greenwichmeantime.com/time-zone/usa/pacific-time/