This is an old revision of the document!
Credits goes to airbender who started writing a very good tutorial. It could downgrade my P3P from 1.11 to 1.7 with success, so I just want to link to his thread.
https://phantompilots.com/threads/downgrading-firmware-on-p3p-from-1-10-9-to-1-7-6.105384/