How do you cancel your green card application with your husband, if you are thinking of filing for divorce?

Just write a letter saying you wish to cancel your green card application. [In accordance with the community guidelines of , the foregoing writing is not "legal advice", nor does it form an attorney-client relationship.]

The first answer missed the question. You file for divorce the same way anyone else does. You establish residency in some jurisdiction and follow that jurisdiction's law. However, you have to be aware of the potential effect on your application.