The standard method for adjusting a guitar neck is by tightening or loosening the truss rod, which is embedded in the neck and determines the neck's angle. However, if your truss rod is loose or has worn-out threads, it can be difficult to accurately adjust your neck in the normal manner. If your neck is bowed, you can use a combination of heat and pressure to realign it.
Remove the strings. Use a string winder to make this job quicker. Attach it to each machine head in turn and rotate the winder so that the strings become slack. Once sufficiently slack, uncoil the top of the string from the barrel of the machine head, snip off the coil with wire trimmers and slide the remaining portion of string out through the bridge.
Lay the guitar on its back atop a work bench. Measure the distance between the bench and the bottom of the guitar neck. Jot down the distance.
Set up two bench clamps approximately 12 inches apart. Adjust both clamps to the same height as the distance between the guitar neck and the bench.
Turn on a clothes iron. Set it to medium and let it warm up while you prepare the guitar.
Wrap an old T-shirt or sheet over the neck of the guitar. It's important to create a barrier between the varnish on the neck and the heat of the iron. The heat is necessary to loosen the adhesive bonding the neck to the neck pocket, but never apply heat directly to your guitar. Wrap a second old T-shirt or sheet around the body of the guitar to protect the paint work from the work bench.
Run the iron gently over the top five or six frets in the area where the neck joins the body for approximately two minutes. The heat partially melts the glue and affords a small amount of movement in the neck joint. The heat also causes the wood to expand.
Place the guitar on its back and position the neck so that it lies between the centre of the two clamps. Gently tighten the clamp nearest to the guitar head to a tension that prevents any movement.
Tighten the second clamp so that it pushes the bottom of the neck downward. This realigns the angle of the neck. Leave the guitar in the clamp overnight. As the glue cools, it will reset with the neck in a straight position.
A guitar-bending machine is designed to apply heat and pressure at the same time to correct bowed necks -- these are expensive and using a clothes iron is a cheaper alternative. This process is for correcting back-bowed necks, the typical symptom of a warped neck or faulty truss rod. In cases where the neck is bowed forward, apply more pressure to the clamp at the top of the neck.