As stated at priceline.com, Priceline has online self-service tools to allow customers to cancel reservations. A common outcome should either be a refund or a credit for future travel, depending on the latest waivers in place.
Cancel your rental car online:
Log into My Trips if you have a Priceline account, or search Find My Trips and select ('View/Cancel Itinerary') next to the reservation you want to cancel;
Look for a 'Cancel' button within your reservation page and read the policies associated with a cancellation. If you’re satisfied with those options, click Cancel;
You should receive an email confirming your request to cancel that may include an update on your refund/credit request. If not, you will receive another email if your cancellation is complete.
Choose your option based on your travel date:
If your travel date is within 72 hours, you can contact the Priceline customer care at 1-80*-774-2354. When you call, they will give you the option of a call back rather than waiting.