How can you treat wrist pain for a teen girl?

Diagnosis. You have to find out why she is having pain. It can be something as easily treatable as tendonitis or it could be a bone problem.
Depends upon cause. Is the teenager active on a smartphone, computer or video games? Is she involved in a sport that requires frequent use of hands? Has she injured her wrist in an other way? Treatment depends upon the reason behind the pain. Have her examined by her doctorr.