The general rule is that you don't. When you made a Best Offer you promised to give the seller an amount of time (there were 3 choices as to how long the maximum time would be) to accept (and pay for it if s/he did) 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 reject your offer, 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.
