Thats not a preset, thats adjustable. You need to turn the steel, hex-nut looking piece in between the dark bit that screws into the gun and the silver bit that your guages and fill nipple are screwed into. Without air in the tank you can do it my hand (but you wont know when you reach 400) but with air in the tank you can use the special spanner that should have come with it or just a big adjustable spanner. The one that comes with it uses those circular indents for leverage.
EDIT- Doh! Parksy beat me to it.