The wrist extensor is essentially the wrist flexor flipped over.
Stretch the affected arm straight out in front of you, but this time keep your forearm and palm facing down. With your other hand, grab your extended hand and slowly pull it down and toward you. Just as you did in the wrist flexor, only bend at the wrist so the rest of your arm stays straight. You should feel the stretch on the top of your arm. Hold the stretch for five seconds then release. Repeat two to four times and then do the exercise on the other side if needed.