You can withdraw your parent's pending visitor visa extension application. You will need their own signature on this withdrawal letter as the application to extend was also filed by them while they were in the US. Or if your parents have authorized you to be their attorney, you can file the application on their behalf but their signature would still be required for this authorization on the withdrawal letter. We recommended generating this letter while your parents are in the US without the place and date. That way, you can use this letter later to withdraw the pending visitor visa extension application if they leave.