Calibrating a home gas oven
First, you need an accurate thermometer that measures average temperature to adjust an oven. For example, a Proaccurate Oven Thermometer, which works by having an oil-filled chamber to average the temperature.

Normally, you can just pull the temperature dial off—the (usually) plastic piece you turn generally pulls right off the actual thermostat, its held on by friction. A gentle prying may be necessary.

After removing the knob, you'll have one of two types of adjustments:


They'll be two screws on the knob itself (in which case, loosen them both to adjust the temperature, it should be marked as to how much to turn it and in which direction—though, if you stare at it long enough, you can figure it out even if its not marked)
There will be one screw inside the shaft on the thermostat. It may be hard to see the screw (it can be at the very end of the hollow shaft). Turn it using a very small screwdriver.


After adjusting the dial, you should re-measure using the thermometer (and possibly adjust again...)

Appliance411 has pictures.