How do you replace water valve on washing machine?

Depends on the brand, but generally you will need to turn off the water supply, unscrew the supply lines from the valve on the back of the machine. On most brands, you will have to pop the top of the machine. there is usually a 5/16 screw holding the valve in place. Remove the screw, disconnect the rubber tube (usuallly a spring clamp) and remove the old valve. Install the new valve, making sure to use the screen washer that should have come with the new valve.