First unscrew the handle nut (A) and then follow the directions under Ryobi 18V Cut-Off Trimmer – Installation Guide. The motor housing with the trim head (B) must also be removed. Then unplug the battery, then remove the wiring harness and then remove the trimhead motor.

Moreover, how do I change the string on my head trimmer?

Remove the head by removing the trim head from trimmer. Insert the new head, then press the trimmer and the new head will fall into place. Use the new head by pressing the trigger. Replace the old head by pressing the head assembly.