How to fix cross-threaded derailleur hanger?
If its a replaceable hanger, simplest option is replace it. 

If not you might be able to repair it with a 10mmx1mm tap. If that does not work, a Helicoil type repair is possible. Its one of those tasks if you are not 100% confident you can do right first time you might be better off handling over to a shop (An engineering shop or well equipped bike shop) as it is possible to make it worse.

Refer Park Tool Basic Thread Concepts