The shaft on a Stihl line trimmer houses the handle, throttle and driveshaft of the trimmer. On the end, the cutting head attaches to the shaft and spins when the driveshaft spins. This flexible shaft can get bent or broken due to rough use or accidental dropping. If just the outer tube is damaged, you'll only need to replace that, but if the driveshaft has also been damaged you'll need to replace that as well.
Unscrew the three screws holding the drive tube to the engine. Take out only the centre screw, which locks the drive tube to the engine. Loosen both screws at the bottom of the throttle handle, but don't remove these screws. Pull the drive tube slowly away from the engine.
Unscrew all of the screws holding the front handle in place on the old drive tube. Remove the screw at the top of the drive tube, which holds the tube to the cutting head. Remove the cutting head from the tube; inspect the gearbox and cutting head and repair if necessary.
Withdraw the driveshaft from inside the drive tube by pulling it out slowly with your fingers. Check the driveshaft; if it's turned blue, it will need to be replaced. Pull the flexible liner from the drive tube and save, if you're only replacing the tube. Save your driveshaft and flexible liner if you will be reusing them.
Push the flexible liner into the new drive tube, lining the shaft up with the cross holes on the tube. Coat the length of the driveshaft with the Stihl gear lubricant intended for hedge trimmers. Apply an even coat over the shaft using your fingers; don't pump grease into the tube or flexible liner.
Push the old driveshaft into the centre of the flexible liner. Push the driveshaft in until it engages the square recess in the flexible liner's gearbox. Check the function of the driveshaft by turning it in the drive tube; if working properly, the gearbox output shaft should rotate, too.
Line up the cutting head with the driveshaft and the drive tube. Insert and tighten the screw to lock the cutting head in place. Refit the handle and tighten all of its screws. Insert the new drive tube into the drive hole on the engine. Tighten the top three screws on the engine. Tighten the two screws on the throttle handle.