You are better off buying a new one. I think it is too much trouble for you to take the weedeater apart, re wire a new cord, and put it back together. Plus, a new cord may cost just as much as a new weedeater and you will save yourself stress.
take the bolts off of the case that contains the pull cord. find out where the string attaches to. spring in drum must be tensioned so that the spring retracts which mean you must rotate the drum to put tension on it. tie the same length piece of sting in.
Remove pull cord cover, and pull out old cord, turn the pully" that the cord was on to wind up the spring, thread new cord into hole in pully tie a knot in end let it rewind cord back in,
You can remove the cover from the weedeater, take the pull cord out and then rewind so it rewinds with the spring. Sometimes the spring will need to be replaced.