If you mean you want to sell it for the "curent bid" amount to the current high bidder, you end the listing choosing that option. But why did you set it up as an auction if you aren't willing to let it run to the end (and bidders who know what they are doing wait until near the end to bid for several reasons).
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. In that case:
To accept it if there has ever been a bid on that auction 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) 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).
If there has never been a bid (and there is at least 12 hours left to go) you can revise the listing to add Best Offer and/or a Buy It Now (but ask yourself why you didn't choose to do so when you set up the listing).
