How do you dismount a 9 mili meter gun?

I'd rather know how you mounted it, to be honest. That aside, I think you mean disassemble, and that's going to depend on the model of gun. It could be, say, a Browning Hi Power, a Sig P226, a Beretta 92, a Glock 17, an Uzi, an MP5, or any of a number of other firearms manufactured in this caliber. The takedown process is going to be different for each model of firearm.