Turn off the fuel pitcock at the end of the fuel line by twisting the small T-handle clockwise until it's snug.
Remove the existing primer bulb by turning the bulb retention ring counterclockwise to detach the ring and bulb from the engine housing. Use a pair of pliers to gently loosen the retention ring, if necessary, and then scrape the surface around the opening to remove any bulb material that might be stuck to the surface around the primer inlet opening.
Insert the new primer bulb into the retention ring. Make certain the small groove around the base of the bulb interlocks with the rim of the retention ring opening.
Thread the retainer ring onto the primer inlet and hand-tighten it. Use a pair of pliers to turn the retention ring another one-eighth turn to fully tighten it.
Turn on the fuel pitcock by twisting the T-handle counterclockwise until it's fully open. Press the new primer bulb a few times to siphon fuel through it to check for leakage and tighten the retention ring, if necessary.