Grips shifts are a type of bicycle shifter common to many mountain and general-purpose bikes. The shifters are convenient to use, since they are located on the inside of the handlebar grip and require only a quick turn of the hand. As the shift mechanism wears out, or you decide to upgrade shifters, the grip shift will need to be replaced.
Shift onto your smallest chain ring if replacing the front shifter or cog if replacing the rear shifter. This will give the shift cable maximum slack.
Use a 5mm Allen wrench to loosen the shift cable anchor bolt on its respective derailleur. If you're unsure where this bolt is located, follow the cable from the shifter to its terminus at the other end.
Use needle-nose pliers to cut the metal crimp from the end of the shift cable and pull the cable from the derailleur.
Open the small cable hatch on the body of the grip shift, grab the head of the cable and pull the cable from the shifter.
Slide the stationary handgrip off the handlebar. Loosen the clamp bolt that locks the shifter to the handlebar and remove the grip shift from the handlebar.
Slide the new grip shift onto the handlebar. Before tightening the clamp bolt, adjust the grip shift so that its barrel adjuster (the little "barrel" at the end) is just below and behind the brake lever. Tighten the clamp bolt when done.
Reinstall the stationary grip beside the grip shift. If necessary, use a small amount of talcum powder on the handlebar during installation. This will help a difficult grip to slide on more easily.
Open the cable hatch on the new grip shift and insert the shift cable. Once the cable emerges from the cable housing at the other end, grab the end of the cable and pull until the head of the cable is seated inside the grip shift. Close the cable hatch.
Shift the new grip shift so that the gear matches the last gear on your old grip shift. This will ensure that the gear and derailleur position match up.
Pull the end of the shift cable taught with needle-nose pliers and tighten the cable anchor bolt. Squeeze a fresh metal crimp onto the end of the cable. These crimps keep cable ends from fraying.