If years of use or just some hard riding have your bike in need of a new grip shift cable. Replacing a grip shift cable is a job that anyone with the ability to use a wrench can perform. Handling this task yourself may even give you the confidence to take on more of your bike's maintenance.
Start by shifting the front derailleur on the bike to the smallest sprocket, and then cut the old cable cap off with the cable cutters. Loosen the pinch bolt enough to take the cable out of the derailleur and cable housing.
Loosen the brake lever on your handlebars and move it aside so that you have plenty of room to get to the grip shifter. If you are replacing the front cable, move the grip shifter so that it is on the lowest gear. If you are replacing the rear cable, move the grip shifter so that it is on the highest gear. Use an Allen wrench to loosen the bolt with the hex fitting that is located on the shifter body. The shifter should now be loose and turn easily on the handlebars.
Note how the cable is wrapped inside the shifter so that you may put the new one in the same way. Open the shift grip housing (it splits in two pieces), and then slide the cable out.
Add a little waterproof grease to the grooves inside the shifter where the cable lies. Take the new cable and slide it into the shifter in the same way as the old one. Put the two pieces of the shifter back together. Put the brake lever back into its original position and tighten it. Move the new shifter cable so that it is under the brake lever and tighten it.
Put some light oil into the cable housings. Run the cable back into the housings and the derailleur pinch bolt. Check that the shifter is on the smallest sprocket, pull the cable tight and tighten the bolt down. Use the cable cutters to cut of any excess cable, leaving about 2 inches beyond the pinch bolt. Put the cable cap back on.
Be careful not to lose or move the small piece of metal that is inside of the shifter. The shifter must have this aligned properly to function correctly.