From the Buy Out Feature (Admin Interface -> Settings -> Listing Settings -> Auctions & Products -> Buy Out Feature) menu:
Enable Buy Out
If enabled implements buy out in auctions.
Always Show Buy Out Button
If enabled, with the Buy Out Feature enabled, the option will remain active even with bids above the reserve price.
Create an auction, enable Buy Out Feature and set a Buy Out Price:
The Buy Out button will be on the right side of the screen.