Allow you Ryobi string trimmer to cool if recently in operation so not to accidentally burn yourself.
Set your Ryobi string trimmer on a table, with the pull-start facing up. Locate and unfasten the screws holding the cord-cover in place on the trimmer using a screwdriver. Put the screws in your pocket to avoid losing them.
Remove by hand both the spring clip and the pawl, a small bar that goes into the notches of the flywheel.
Pull the freed cord-cover off, exposing the cord wrapped around the rotor. Unwrap the cord and cut it with diagonal pliers, freeing it from the rotor. Then, pull it out of the cover. Set the old cord and handle on the table.
Untie or cut the knot on the rotor and remove the stub of cord left on the rotor and dispose of it.
Pick the cover up and pull the old cord out. Dispose of the cord, then pop the cap off the handle with your fingernails or use a flathead screwdriver.
Retrieve the replacement cord. Feed it through the handle and through the cover, leaving slack coming out of the handle. Tie a knot in the end and pull the knot down into the handle, then replace the cap on the handle.
Grasp the free end of the cord and knot it into the rotor where the old knot was located. Put the pawl and spring clip back in place.
Grab the rotor by hand and turn it counterclockwise several times until it no longer turns, then allow the rotor to spin in the opposite direction, wrapping the new cord around the rotor.
Put the cover back on your Ryobi string trimmer, fastening the screws in place with a screwdriver.