Once your listing has been launched, you can update your pricing in limited circumstances from Edit listing > Set your pricing in your listing.
In an auction listing, the starting bid price, reserve price, and Buy It Now price can all be updated after the listing is live if no bids have been placed. The starting bid price cannot be changed once a bid has been placed. The reserve price can be lowered once bids have been placed to $1 above the highest current bid amount, but not raised once it has been met.
In a classified listing, the asking price can be changed at any time.