It is too late to do any significant revision to the listing once it has ever had a bid. You would need to end the listing early without a winner (cancelling the existing bids) which involves a fee of the Final Value Fee % for that listing times the highest cancelled bid (the hidden maximum, not the showing "current bid") which is waived once per calendar year (don't know if that is automatic or you have to contact Customer Service) and then relist correctly (I'm guessing you meant to list fixed-price with no auction).
ETA: Or just let it run and hope for more than the BIN price but take the loss if it's less.
