Use the Revise Item link, top left lf listing and mmediately under the heading "Choose how you'd like to sell your item" there are the options to choose from - Online Auction or Fixed Price.
However, it may depend on how you listed in the first place. If you used the quick sell tool the option may not be there so you'd have to end early (via dropdown arrow, far right of listing on your selling page) and relist using the advanced sell form (link to switch forms is top right after clicking on Sell).
