There are no core rules for retraining weapon proficiencies.
As a rules extension, a weapon proficiency can be exchanged for another in 1 month at the cost of a cumulative 250 gp times the highest class level of the level of the character (1st = 250, 2nd = 750, 3rd = 1500, 4th = 2500, etc.).Ex As with regular training, the character cannot perform any other activity during this time; if they do, all progress and expense is lost.