You can't.
It's a commitment.
You can contact the seller, explain that you don't want the item and ask for a Mutual Cancellation.
She doesn't have to agree.
If she does, accept promptly and leave positive feedback. Five Stars Detailed Seller Ratings for Communication would be nice too.
If she does not, she may open an Unpaid Item Dispute.
If you do not pay then, you get a Strike on your account that will make it much more difficult to bid on eBay in future.
