DUMLdore is a Windows app that can transfer firmware to/from a DJI product. You can download and read detailed instructions at the GIT repository here

• Aircraft:
  • P4
  • P4 Pro
  • Mavic
  • Spark
  • I2
• Remote Control
  • Mavic
• Goggles
  • DJI

• DOWNLOAD THE ENTIRE ZIP FILE NOT JUST THE EXE
• Make sure you have at least 50% battery remaining before you begin
• Make sure Assistant2 is not running
• Turn on your device (Aircraft / RC / Goggles) and allow it to fully start up before connecting to PC. (Wait about 30-60 seconds)
• Plug in USB cable from the aircraft/RC to the PC. Make sure you are using a good DATA cable, NOT a phone charging cable.
• Start DUMLdore.exe
• Confirm connectivity
  • If DUMLdore does NOT detect your device, close DUMLdore, run Assistant and let Assistant detect the device.
  • If Assistant DOES NOT detect your device, DUMLdore will not work.
  • Once Assistant detects your device, CLOSE assistant before running DUMLdore
• Use DUMLdore
  • LOAD: Will load a firmware file to flash. Some error checking is done to make sure it is valid. It is not perfect and up to the user to make sure it is.
  • FLASH: Upload the firmware file to the device and begin the upgrade/downgrade.
  • BACKUP: Will make a flashable firmware backup of the currently connected device if applicable. It will save the dji_system.bin file into the current directory. It may take some time. Please be patient as the status may take a few moments to update on some of the files. If you are making a backup of your aircraft and your remote controller, rename or move the 1st backup file into a folder before running the backup on the next device.

You can see available firmware here or directly from GIThub

Thanks to hostile, the_lord, p0v and many others for the information :)

-jezzab (copied from his Git, with additions)