Cancelling returns the piece to your wallet, free of any sale. It is a transaction on chain, like listing was, so it costs and it takes a moment.
The button
The open padlock, where the closed one was when you listed it. Same place — your wallet, or the piece's page — and it only appears on pieces of yours that are currently for sale.
What it costs
80,000 sats, fixed, whatever the piece is worth.
Worth knowing before you list: a price you are not sure about costs you twice — once to list, once to take it back.
Cancelling is not instant, and the piece is still for sale meanwhile
Your cancel is queued, signed and broadcast. Until it confirms, the listing is live and anybody can buy at the price you set.
That is not a flaw in the cancel: a listing is a contract on the chain, and nothing can un-offer it retroactively. If it sells while your cancel is in flight, the sale is valid and you are paid the price you listed.
So do not lower a price by cancelling first. If the piece sells during the gap, it sells at the old price. That gap is your risk to take.
Do not press it twice
The button locks itself until the request resolves. If it looks stuck, look at the jobs panel before clicking again — two cancels mean two fees.
How to know it worked
The piece stops showing as for sale in your wallet and disappears from the market listings. If it is gone from the market but your wallet still shows it as listed, that is the display catching up: My balance or my wallet looks wrong.
When you have to cancel first
If somebody offers on a piece you already have listed, you cannot accept without cancelling: the live listing has a different price from the one you agreed. Accept an offer explains why.