First of all, your engine needs to be properly tuned to shift correctly. If it's tuned and warmed up, you pop the rubber cap and rotate the trans till you see the center grub screw and one off center. It's the one OFF center that will change your shift point. Only turn it 1/8 turn at a time, then run it and readjust it slightly in either direction, if necessary.
It's like a clock. Turn it clockwise and it's later, counter clockwise and it's earlier. Be careful backing it up. If you go too far it will come out and you'll have to take the trans apart.
Adjusting the 3rd gear shift point (If you know you have a 3rd gear) requires the trans to be opened and the adjustment done the same way, but from the inside.