Be careful not to drop the spring and washer. (Figure 2.)

Sometimes its easier if you remove the wire bracket screw also but it is not necessary. (Figure 2.)

Remove the screw the holds the button assembly (Figure 1,)

Pull the button assembly out. I used a needle nose pliers to lift out the switch.

Disassembly: (Figure 3.) Carefully pry the spring bracket in the direction of the red arrow.

Spread it just enough to release the spring plate in the slot and no more.

Be careful not to drop the spring.

If you look closely you can see that the small end of the spring is on the tab of the button

and the large end of the spring rests on the spring plate.

The spring plate is soldered to the blue and white wire

that carries the ground to the starter solenoid.

Remove the old button.

Reassembly: Install the new aluminum button into the spring bracket.

Install the spring small end toward the button and large end toward the spring plate.

Now squeeze the spring bracket in the direction of the blue arrow ever so slightly. 

Now push the spring plate toward the spring and into the slot on the spring bracket.

If you squeezed the spring bracket correctly the spring plate will snap into place.

A pair of needle nose pliers can help here.  

Now simply reinstall the button assembly into the switch assembly and reinstall the light switch

if you have one on your bike.

Reinstall the throttle cable and the two screws with cable bracket.