The simple answer is - drain out the old primary oil and put in the new required amount of 1 quart.

By the left front footpeg on the motor is a keyhole shaped cover with 3 screws. Remove these screws to remove this cover. Make sure your bike is upright, as oil will spill out if you are on the kickstand. Use this diagram to fill up the oil level till it reaches the dotted line like shown: http://www.badweatherbikers.com/buell/messages/3842/408472.jpg