Go here:
http://offer.ebay.ca/ws/eBayISAPI.dll?RetractBidShow
You will need the listing number which is on the right side of the page above the description.
It will be easier to do this on a desktop or laptop than on a phone, because you can see the instructions better.
It would be polite to use Contact Seller to explain why you cancelled your bid. Most sellers would rather have a bid cancelled than try to chase an unwilling buyer with an Unpaid Item Dispute, but knowing what the problem is makes him a better seller.