I was wondering if there was a way to reset the achievements in Postal 2? I would like to reset them so I can replay the game and try to unlock them again. Thank you.

Try deleting save files, they should be here - <path-to-game>\Save

BOOM instant answer!

Achievements are stored in Postal2.ini and ParadiseLost.ini, depending on bUnlocked=True & bUnlocked=False per Achievement. Cannot see which is which, you have to count.