Click on the number of bids in the listing and scroll to the bottom is a link to retract called "learn more".
Once you retract you will be asked to enter the correct amount.
Remember to use a fullstop, a comma doesn't register as a decimal point in ebay.
Yes, but you have to retract the original bid and then re-enter your bid at the lower amount, and it can only be done if there is more than 12 hours to go on the auction, or if there is less, within one hour of placing the bid.
This is the bid retraction form (I believe you can also access by viewing the bidding history on the auction in question) : http://offer.ebay.com.au/ws/eBayISAPI.dll?RetractBidShow
You will need the item number, located on the right of the listing underneath the seller's details but before the description.
Edit: missed it by that >...< much. 
