The update procedure for the P6 is the same as the one for the Micromonsta, except that, to enter the USB update mode, you need to enter the setup Menu (SHIFT+GLOBALS) and press encoder 3 button. LCD will say “USB DFU MODE ACTIVATED”.

Find the Micromonsta update tutorial here:
https://www.audiothingies.com/micromonsta/firmware-update/

IMPORTANT: the process is the same, but do not send a MicroMonsta firmware to the P6, it won’t work and will brick your unit.
Make sure you send a P6 firmware