The way that you change your bid depends on whether you’d like to raise it or lower it.
You can raise your maximum bid at any time before the end of a listing. To do this, simply place another bid on the auction at a higher amount.
When you are the current high bidder in a reserve auction and the reserve has not been met, your bid may be raised if you place another bid that meets the reserve amount. This allows the listing to end successfully.
In order to lower a bid you will need to retract your original bid first and then place your lower bid.
There are only a few exceptional circumstances under which you may retract a bid. These are if:
You made a typographical error and entered the wrong bid amount. For instance, you bid $99.50 instead of $9.95. (Changing your mind is not considered a typographical error). If this happens you must retract the erroneous bid before you re-enter the correct bid amount. If you do not place another bid, the retraction will be in breach of eBay's policy and could result in your suspension.
The description of an item you have bid on has changed significantly.
You can't reach the seller. This means that you tried calling the seller but his or her phone number doesn't work, or you have tried emailing a message to the seller and it comes back undeliverable.
Please review the retraction guidelines if you need to retract your bid.