You'll probably have to replace the whole shifter knob. I haven't ever tried to take the button mechanism apart, but I don't think that separate parts are available to replace just the button.
To remove the entire knob, with the engine off, move the shifter knob all the way to the back to the 1 position. On the forward side of the shifter knob there's small clip that can be removed with a small screwdriver. Pull the clip towards the front of the car to remove it and lift off the shifter knob.
A dealer should be able to sell you a new one that will match your interior color, but you'll need to provide him with your VIN so that he can identify the color.