Credits goes to airbender who started writing a very good tutorial on the phantom pilots p3 forums: https://phantompilots.com/threads/downgrading-firmware-on-p3p-from-1-10-9-to-1-7-6.105384/
The process is the same for Phantom 3 and Inspire 1, just different files.
To downgrade the firmware, you need to to copy the prior version of firmware BIN file no the SD card with a special firmware debug file. The existance of the firmware debug file will allow the aircraft to skip any version checks and downgrade to the prior version. You do this at your own risk, there are possibilities the downgrade could freeze or cause problems. Only do this if you know what you are doing and never skip versions.
Warning: The gimbal mainboard may get hot during a few firmware flashes. The fan on the gimbal from P3P is not a great help. So it’s better to pretend overheating during multiple firmware flashes to simply take a additional fan for cooling and take breaks.
You can download all available Phantom 3 or Inspire 1 firmware from the DankDownloader Tool: http://dankdronedownloader.co.uk/DDD2/app/
Once you have downloaded the firmare, copy the .BIN file to your SD card.
Now, you need to make or secure a firmware debug file. This file is a text file, with no extension and no contents in the file. It is effectively a zero bytes file with a special filename. The filename needs to match your aircraft model. Below are the firmware downgrade filenames:
To create the file, create a new file as a text file. Rename the file to one of the names noted above, do not include a file extension. The contents of the file shall also be blank (that is, no contents). Save the file. Copy to SD card with firmware you want to install and then power on the aircraft. ONLY USE THIS IF YOU KNOW WHAT YOU ARE DOING!
Zip with all debug files is also available: https://github.com/digdat0/P3I1_FWProject/raw/master/Debug%20Files/Debug%20Files.zip