Please watch this video to understand the changing of the XM203 Rate of Fire.

XM203 Rate of Fire Video

The up and down key will change the RPM (Rounds Per Minute) 10 RPM per press.

For example, pressing "Up" 10 times will change the current RPM of 850 to 950.

Once the Max or Min is reached, there will be an audible tone and a solid orange light for 1 second indicating you can not go up or down any further.