Spanner wrench (with flat tips)
There are two basic types of airbrush actions: single and double.
Without going into too much detail, single-action means that the trigger on the airbrush only controls the amount of paint being released; the air flow is fixed.
Double-action airbrush triggers control both the amount of paint, and the amount of air—in most cases, press down for air, pull back for paint.
With a weak air compressor or pump, sometimes a double-action airbrush is not needed, and a single-action is far simpler and more convenient.
This is the valve on my double-action airbrush that controls the amount of air going in. The spring-loaded pin in the center is actuated by the airbrush trigger. Press down the trigger and the pin moves downwards, opening the valve.
You can see the pin more clearly in this picture, along with the retaining ring inside the shell.
Using the spanner wrench, unscrew the retaining ring slowly. Remember that there is a spring behind, so take care not to have the whole pin/spring/retainer flying off.
When you’re done, you should have these two parts separated. Place the removed parts in a safe place so that you can replace them should you decide to reverse the conversion.
P.S. If you don’t have a spanner wrench handy, a pair of tweezers or dividers will probably work too.