If the deal is still under negotiation (the banknote is orange):
Go to the banknote you’ve made a bid to; at the bottom under “Actions”, you will see a Cancel button.
You can access the banknote by either finding it directly on the map (it’ll be black), or by going through the Dashboard (Menu->Dashboard), and finding it under My Bids
If the deal is still confirmed (the banknote is black):
You will have to log a Dispute in order to bring to our Customer Happiness team to handle the cancellation. Go to the banknote you're confirmed into; tap on Evaluate, and highlight one star for the rating. A button "Log a Dispute" will show - click that.