Electric guitars and steel string acoustic guitars have truss rods installed in the necks. The truss rod is a metal bar that runs through the entire length of the neck. This makes it possible to make minor adjustments to the curvature of the neck. Guitars are affected by humidity, moisture, and temperature which may cause the neck to go out of adjustment. A typical indication that a truss rod needs adjustment is buzzing strings higher up the fretboard. The truss rods in Epiphone guitars are adjusted the same as the truss rods in other acoustic and electric guitars.
Inspect the curvature of the neck. A simple way to do this is to look down the guitar neck from the headstock towards the body of the guitar using the strings as a point of reference. If the neck is straight, the distance between the fretboard and the bottom of the strings should remain the same the length of the guitar neck. A second, and more precise method, is to place a capo on the first fret. Press the sixth string down on the last fret where the neck connects to the body of the guitar. Look carefully at the 12th fret. There should be a small amount of leverage between the string and fret. If the string is laying on the fret, the truss rod needs to be adjusted.
Loosen the strings to relieve the tension on the neck.
Locate the opening for the truss rod. The truss rods on most Epiphone electric guitars are accessible in the headstock directly above the nut. The truss rods on Epiphone acoustic guitars are accessible through the sound hole where the neck joins the body of the guitar.
Adjust the truss rod with an 3 mm (1/8 inch) Allen key. Insert the Allen wrench into the slot in the truss rod. Turn the truss rod to the right to tighten it and straighten the neck. Turn the truss rod to the left to loosen it and increase the bow of the neck. The neck should be almost straight with very slight backwards angle. Give the truss rod a quarter turn and inspect the neck before proceeding. Stop adjusting the truss rod when the neck is straight. Test and play the guitar.
It is possible to damage the guitar by turning the truss rod too far. If you are inexperienced, it is a good idea to ask a more experienced friend to supervise you.