Whether you're the buyer or the seller, you can cancel an offer by sending the other person a message on OfferUp. There currently isn't a cancel button for offers, so messaging the other party directly is the recommended way to handle it.
How buyers can retract an offer
If you made an offer and want to retract it, open your conversation with the seller and send a clear, polite message letting them know you'd like to cancel the offer.
How sellers can decline an offer
If you received an offer and don't want to accept it, open your conversation with the buyer and let them know you're declining or canceling the offer. Keep your message short, polite, and direct. If the item is still available, you can also let the buyer know they're welcome to make another offer.