im not too familiar with the tmaxx tranny, but im sure its not too different than a savage tranny,

on the case, you should see a little rubber plug, take that plug off and look for the adjsutment screw. clockwise will make it shift later, countercockwise will make it shift earlier.

but before you go screwing with that. take the truck off the ground so the wheels spin freely. and give it throttle gradually. listen closely for the shift. and then adjust it in whichever direction it needs to be adjusted.