You can only edit your Auction up until the Reserve Price has been met. If there's no Reserve Price set, then the Auction can be edited up until the first bid has been placed.
To edit an Auction, simply go to your dashboard and navigate to Auctions in the menu on the left. Use the drop down to select Auctions.
Then click on the blue edit button next to your Auction.
This will open your Auction details where you can make changes to the applicable field(s) and proceed through the steps by clicking Next.
When you have finished making changes, you will have to accept the Final Value Fees and then click Submit at the bottom of the final tab (Media) and your Auction will be updated.