Shut off the water valves under the sink in the kitchen. This prevents water from running through the faucet head.
Unscrew the Allen head set screw located either in the back of the faucet handle or bottom front of the handle (some Delta faucets have a screw in front and others in back). Pull the handle up and off the faucet after loosening this screw.
Push the washer ring on top of the faucet head with the end of a screwdriver or a pair of pliers to loosen it slightly. Remove the dome cap (rounded piece that was under the faucet handle) by turning it counterclockwise with pliers.
Take out the ball valve and the two seat springs in the faucet head. These are round rubber washers with springs attached to them. Lift them out with the end of the Allen wrench.
Replace the seat springs with new ones and replace the ball valve with a new, matching part.
Put the washer back on and tighten it with the pliers. Attach and tighten the dome cap to secure the ball valve. Replace the handle and tighten the set screw with the Allen wrench.