Been there and done that. Please consider using eye safety glasses.
If you are up to a challenge, first purchase a length of nylon rope about the same diameter of the present pull rope about six feet in length. Next, remove the top of the engine cover by removing a small number of screw/bolts found along the sides of the cover. On the underside of this cover you will find the housing and recoil spring. Examine the operation of this spring by pulling carefully on the pull rope just a bit. If the spring seems to be still intact and not broken will will not need to replace it. DO NOT ATTEMPT TO REMOVE THE COIL! but first find some flexible small wire or use string to tie this coil to itself. (If you fail to carefully tie the coil together in 3-5 points around the coil you will be unable to replace the coil within the housing.)
After you have securely tied the coil tightly together you can then remove it from the cover housing and cut away the old pull cord. Now install the new pull rope with good knots. Don't worry about the length of the rope just yet, but wrap as much as the housing will allow. Be certain to wrap the rope in the same direction as the old rope was wrapped. Next, replace the coil spring and pull rope back within the housing and, after ensuring it is securely within the housing, then cut the small wires or string that has secured the coil. Now, go get a beer and pat yourself on the back. Money saved and the repair should last for several years more.
If the tasks set forth above seem too challenging, you will avoid a great deal of frustration by visiting a repair shop. The tension within the recoil spring is larger than you can imagine and it is a danger to you if it is not handled with care. If it comes free of the ties mentioned above then go get two beers and a band aid.
take the recoil off and use a small flat screw driver near where the rope is coming in. Pull on the rope (lightly)while you loosen it with the screw driver. Stick the screw driver between the rope and kind of cross it over.....wiggle it around..until it loosens up. Make sure you start with the very end of the rope coming from the pull cord handle and work your way around. More than likely it will be stuck right at the beginning
you would take off the cover and expose the recoil spring/shaft then detach the string and put the new one on
String got under shaft
pop the spool out wind string in direction of arrow, replace
http://72.3.225.179/manuals/545117526e.pdf
On most of them you hold the top part with one hand and turn the bummer/spool clockwise (looking from the top)and pull it straight down/outward
Trimmers, Weedeater, Weedwhacker, String trimmers.
get a new one
http://www.stihllibrary.com/pdf/trimmerheads/AUTOCUT_25-2_101001.pdf Go to this link and all the string replacement guides are there.
To thread a weed eater is very simple and easy just takes a few steps. First you have to get the string and tie a knot in one end and then wrap it around into the other end of the weed eater and replace then cover then it should be go to go.
Unplug the weedeater from electrical source. Turn the weedeater upside down and remove the black knob on the bottom. If the black knob won't come off you can carefully use a large pipe wrench and turn it counter clockwise to loosen it. Be careful not to lose the slotted metal ring between the black knob and the trimmer spool. Remove the spool and clean it off. There is an arrow showing you where to insert the string. Start pushing the line into the spool going counter clock wise. Be sure to use only .065 ROUND string. Put the spool back on the spindle yellow side up. Hold the slotted metal ring with the slot facing up. Insert the string into the bottom of the ring and put the ring in the recessed circle on the spool, (yellow side). Pull the string over the slot and hold it there. Screw the black knob back onto the spindle, making sure the string is in the slot on the metal ring. Tighten the knob. You are now ready to go.
remove head pull shaft out
you pull the plugs and spray generous amount of W-D 40 in.