If you are the seller https://www.ebay.com/bfl/cancelbid
If you are a bidder the general rule is that you don't. When you placed your bid you promised to keep that bid on that item (and pay for it if you won) absent a very few very specific extraordinary circumstances. If you think your reason might be such an extraordinary circumstance, let us know specifically what it is (the reasons are very narrowly defined), when you bid, and when the auction ends and we can give you an accurate answer to your question (or an alternative solution, if there is one, to your problem). Or you may contact the seller and beg him/her to cancel your bid before the auction ends, but s/he is under no obligation to do so nor to acknowledge the request, and neither not responding nor responding that it's OK with him/her is among the valid reasons. If s/he claims not to know how to cancel send him/her the link at the top of this post.
