Visit an authorized Ryobi dealer and acquire a new attachment head assembly (Ryobi part number 180897).
Ryobi Outdoor Products is a well known manufacturer of gasoline-powered lawn maintenance equipment. One popular item the company produces is its complete line of string trimmers. If the cutting attachment head on your trimmer has been damaged or is badly worn from use, you should remove it from the unit and acquire a replacement. Doing so will only take a few minutes of you time, and possibly one common tool.
Invert the trimmer, and stand it vertically with the cutting head at the top.
Loosen the "bump knob" or "bump head" by unscrewing it using a counterclockwise turning motion. If it resists removal, grip it with the pliers for more leverage. Remove it and set it aside.
Remove the inner string reel from the interior of the cutting head and set it aside as well. Avoid damaging or misplacing the inner reel spring. After the inner reel is removed, the lower section of the engine shaft will be fully exposed.
Grasp the shaft housing firmly with your left hand. With your right hand, wiggle the cutting head while gently pulling it upward. Continue this motion, until it slips off the large hex nut it nestles on.