Replacing a string on a weed wacker can be challenging. It is difficult to describe and hands on practice is probably necessary. This describes the replacement on a 9 July 2019 Maruyama BT23L Weed Wacker. The internet videos are abysmal.
Two real heads make the job relatively simple. After removing the head from the unit and practicing not installed is probably prudent.
Remove head. This is a left hand thread
Remove cover to expose inside of real. A straight screwdriver is probably needed for some extra force.
Remove any remnant of old string by pulling one end.
New string 0.095 diameter and about 12 feet long.
Push the end of the new string through the reel, this can take some effort other times it goes through easily.
Make the ends of the string even.
Wind the top channel clockwise. This is easy since there are no obstacles.
Wind the bottom channel clockwise. The various posts are in the way and some care is required.
Place the ends of the two string in the dispensing slots opposite each other.
Study each channel and make sure the strings are not crossing. Adjust and move if necessary.
Practice a few times and get a demo from a winder if possible. Replace the reel cover.