Updating the User profile property using REST API

User profile properties from client APIs are read-only (except the profile picture, which you can change by using the PeopleManager.setMyProfilePicture method). If you want to change other user profile properties, you must use the server object model. For more information about working with user profiles

According to MSDN,

But stillUser profile properties are now writable via CSOM.

http://blogs.msdn.com/b/vesku/archive/2014/11/07/sharepoint-user-profile-properties-now-writable-with-csom.aspx

Please check this out.