Go under the sink and turn off the two water shut-off valves. Go back to the faucet and locate the spout bonnet nut.
The spout bonnet nut is the sleeve at the lower part of the spout. Turn the spout bonnet nut counterclockwise with your hand to release the spout.
Slide the bonnet nut up the neck of the spout, and pull the spout out of the retainer in the faucet body.
Turn the retainer counterclockwise with your fingers, and remove it from the faucet body. Roll the O-ring off of the diverter assembly housing.
The diverter sits in the center of the faucet. The O-ring is under the lip of the diverter housing.
Place the replacement O-ring over the diverter housing. Ensure that the ring seats under the housing lip.
Thread the spout retainer back over the diverter housing until it is hand-tight.
Insert the spout into the retainer until the slot on the bottom of the spout engages the tab inside the retainer. Slide the bonnet nut down the spout neck, and thread the bonnet nut over the diverter housing until the nut is hand-tight.
Turn on the water supply under the sink.