A bid is not amendable (except one special case where eBay automatically lowers the hidden maximum of the leader's bid if the seller lowers the Reserve Price in certain situations).
You can raise your overall maximum (as opposed to the hidden maximum for that bid) by rebidding.
Unless you made a typo in entering the bid amount, it is against eBay policy to retract for the purpose of lowering your maximum. The only other valid reason to retract is if the seller added language that significantly changed the description of the item after you bid.
You can message the seller and ask him/her to cancel your bid (but don't count on being able to rebid since sellers who know what they are doing block bidders who retract or ask to cancel) but s/he is under no duty to do so nor to respond, and neither a response that it's OK for you to retract or s/he doesn't know how to cancel or no response are valid reasons to retract.
