That depends. What do you want to change? 1st to 2nd or 2nd to 3rd?
To change 1st->2nd, you use the side hole in the trans case. Rotate the spur until you see the hole in the gear housing, then hold the spur and rotate the tires until you see the adjustment screw. Then you turn the screw in to make it shift later, turn the screw out to make it shift earlier. 1/4 turn increments, then run it and repeat as needed.
To adjust the 2nd->3rd, you have to take the top of the trans off, take out the two speed assembly, take the stupid thing apart and adjust. Put it back together and hope you don't have to repeat!
The manual gives you parts explosions with photo's to show you how and what to adjust.