Check out this PDF that explains how a guitar truss rod works and how to adjust it. This will work on adjusting the neck of a electric or acoustic guitar, bass, guitar, banjo and other similar instruments.
Proper guitar neck adjustment is a part of regular guitar maintenance. The wood of neck can react to several environmental factors like humidity and temperature. The good news is a neck adjustment is a relatively easy fix and will help put your instrument in its best playing condition. If you think of changing guitar strings is like changing the oil in your car, then a neck adjustment is like a tire alignment.