Adjusting the neck
Eyeball your neck from the headstock end. If the neck is dipped or over bent, locate your truss rod bolt (most commonly located on the inside of the sound hole just below the neck) and use an Allen key to adjust. If the neck is dipped (lower in the middle than either end) turn the key clockwise. If it's over-bent (high in the middle than either end) turn anti-clockwise. Never force the truss rod if it's tight.