The little doorbell button outside your door gets a lot of abuse over time from visitors. Some guests might press it too hard or multiple times, while others hit it. The abuse, along with the light bulb inside the light going on and off every time it is pushed, can wear the button out. If it is no longer working correctly or is not lit up, it is best to replace the button.
Turn the power off to the doorbell by flipping the switch in the breaker box.
Unscrew the button plate from the door frame or wall with a drill or screwdriver.
Remove the wires from the back of the plate that connect the button to the house wiring. They may be held to the button wiring by a twisted connection and some wire nuts or by metal screws. Untwist or unscrew to remove.
Attach the new button's wiring to the house wiring in the same manner that the other was removed.
Twist the like wires together, or wrap the wires around the screws on the back of the button plate the way the others were removed. Use wire strippers if the wiring is coated with too much Teflon. Expose about 1/4 inch of wiring to do the connection. Wires are color-coded for easy recognition. White is neutral, black is live and green or copper is ground. You may not have all three.
Place the button plate over the screw holes and reattach.
Turn the power back on.