It depends on your model. You can visit the PIVOTRIM website and they show how to install their head on most units which includes how to take your old one off. Their website is http://www.pivotrim.com Otherwise you need to look at your operators manual or call the manufacturer. Good Luck!
sthilweedeaterhead number f5556
The manual can be found at the link below.
stick a flat head screw driver in the slot above head and turn counter clock wise.
i need to replace the flex shaft in my weedeater. how do i do it?
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
pop the spool out wind string in direction of arrow, replace
remove head pull shaft out
There's either a steel or flexible shaft inside the tube between the engine and the head. That shaft is broken. Or if it's solid, it rusted inside the head
To change the head on a Stihl weedeater, first ensure the trimmer is turned off and disconnect the spark plug for safety. Use a wrench to remove the old cutting head by turning it counterclockwise. Align the new head with the spool, threading the line as per the manufacturer's instructions, and secure it in place by tightening it clockwise. Finally, reconnect the spark plug and you're ready to go.
Your local hardware store should be able to tell you how particularly if they sell the same machine
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.