If you have a message from eBay that you got a Best Offer on it, that is out of date. Best Offer ends on the first bid and all pending offers/counteroffers immediately automatically expire.
If you mean you got a "Message from a member" containing an offer, you can (if there are least 12 hours left to run) but it is usually not in the seller's best interest to accept such unsolicited offers. They are frequently made by scammers but even at best are made because the offeror a) doubts that the amount would win if the auction goes to the end and/or b) is 1) chronically and/or 2) situationally impatient. To accept it you have to cancel all bids and end the listing early without a winner (ticking off your existing and prospective bidders and incurring an extra fee of the FVF% for that item times the highest cancelled bid--the HIDDEN maximum, not the "current bid" that is displayed) and then relist the item in a manner that allows the offeror to buy it properly at the correct price, with no guarantee that s/he will do so (it is likely s/he has made multiple such offers).
