Let's make it public, maybe it will help others too.
How to change PSI level in BMS
1) First of all you need a cable
http://www.burgertuning.com/USB_Cable_Extension.html
2) You will need a windows laptop (doesn't matter if it's windows xp, vista, 7 etc.)
3) Then go ahead and download JB3 software right here
http://burgertuning.com/jb3_interface_062312.zip
My BMS box was placed here, the plastic cover is really easy to lift by hand (no tools needed)
When you find the BMS box, connect it to pc
after you connect your BMS box to your PC, hit the start engine button once, in your beautiful m6.
Now, go to your laptop; first click on usb_drivers.exe; Wait until it's done, then click on setup.exe
After clicking on setup.exe, go to settings, here you will need to set the right Com Port number. To find it out, right click on "my computer" then "device manager" then click on the little arrow beside "ports". if it says COM4 you write port 4 in settings, if it says PORT1 you write port 1 in settings etc.
When you done with ports, everything is connected and good to go, click on File/Connect
Now go to user adjustment. And change "boost over stock" from 2.25 to 2.5, then click Save. Don't change other values!
You're done! Now you have to disconnect properly. Click on file/disconnect. Turn off your car by clicking on "start/stop engine". Disconnect your PC and everything, put the BMS box back, and that's it.
Go for a test drive to make sure everything is good.