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