Replacing the line in a Ryobi string trimmer is an inexpensive job that can be done relatively quickly with just a few tools. Most models need 0.80-inch diameter monofilament replacement line, which is readily available at most garden centers and home improvement retailers. It won't take long to get your trimmer up and running again.
Detach the sparkplug wire to make sure the unit will not start while you are working on it. Models powered by electricity should be disconnected from the power source. Lay the trimmer on a flat spot in your yard, driveway or garage so you can work on the power head.
Remove the spool cap and bump knob by unscrewing the cap. This will expose the inner spool containing the trimmer line. Clean any debris from the power head, using a towel or a stiff brush. Throw away any string that might remain on the inner spool.
Measure 9 feet of your new trimmer cord with a tape measure and snip off the end with your snippers. Position the end of the trimmer cord in the inner slot of the inner spool and start winding the monofilament line counterclockwise. The line should be kept tight and even on the spool. Stop winding when you have 6 to 8 inches of line left. Hold the inner spool so that the cord will not unwind.
Guide the end of the trimmer line through the hole in the outer spool and push the inner spool into the outer spool, trying not to allow the new line to unwind. Replace the bump knob and the cap on the outer spool and reattach it to the power head.
Reconnect the sparkplug wire or plug the unit back into the electrical outlet.