The Shimano SL-RS40 "Tourney Revo" is a three-speed gear twist shifter with handlebar grip mount. Just as with any other parts on a bike, cables from the SL-RS40 may need to be replaced over time from heavy usage, if they stretch or are damaged. Replacement cables can be purchased at a bike shop or online and a cyclist can do the replacement job themselves at home or leave the bike at the shop.
Shift down to the hardest gear.
Take a Phillips-head screwdriver and loosen the plastic screw on your shifter.
Remove the plastic screw.
Push the cable completely out of the shifter. The cable end should pop out where the screw was.
Keep the shifter in the most difficult gear.
Slide the new cable through where the old cable came out.
Tighten the screw back down tightly with the Phillips head.
Hook up the new cable into the derailleur from which you removed the old cable.
Inspect the cables on your bike often to make sure this isn't damage or too much wear and tear.