Drain the fuel tank. Remove the plug and drain any remaining fuel into a proper gas can. Replace the plug.
Locate the fuel line. Look for a rubber hose that runs from the side of the fuel tank to the engine. The fuel line will normally be located on the inner side of the fuel tank that is closest to the engine.
Find the brass fitting at the end of the fuel line that inserts into the fuel tank. Remove any metal clips that hold the fuel line to the tank. Hold the brass fitting on both sides, and pull it straight out from the grommet that holds it to the fuel tank. Pull out all of the fuel line that is inside the fuel tank. Pry off the grommet with a flathead screwdriver.
Remove any other metal clips that attach the fuel line to the housing of the lawnmower or to the engine.
Pull out the brass fitting that attaches the fuel line to the engine. Pry the old grommet off with the edge of a flathead screwdriver. Lift the old fuel line away from the lawn mower.
Hold the old fuel line next to your new fuel line to make sure they are the same lengths. Remove one of the brass fitting and trim the fuel line with scissors if you need to shorten it.
Insert the new section of fuel line that goes inside the fuel tank. Feed the hose all of the way into the tank until you reach the other end with the grommet. Push the grommet firmly into place to seal the hose into the tank. Look inside the tank and make sure the fuel line is not coiled or bunched up. Use a screwdriver to uncoil it if you need to.
Push the brass fitting through the grommet and into the fuel tank. Attach the other section of fuel line to the fitting. Clip the fuel line to the tank with the metal clip you removed earlier.
Insert the other grommet into the engine where the old grommet had been. Push the other brass fitting into the grommet and into the engine. Attach the free end of the fuel line to the brass fitting.
Attach the new fuel line to the lawn mower using the metal clips you removed earlier.