Token owners who have their NFTs listed as a reserve auction are able to cancel the auction at anytime before the reserve auction has started. Once a reserve auction has begun there is no way to undo or cancel.
Click onto the NFTs page
Click the "Cancel Auction" button under the NFT
Confirm your auction cancellation by clicking the red "Cancel Auction"
Your wallet will prompt you to confirm a gas fee to complete the process.
You will see a confirmation screen to confirm your auction has been cancelled.