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
  • Spark
• Goggles
  • DJI

As of 2.11 incorporates DUMLRacer to allow downgrading of Mavic FW 1.04.0100

• 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 / Goggles) and allow it to fully start up before connecting to PC. (Wait about 30-60 seconds)
• Plug in USB cable from the aircraft to the PC. Make sure you are using a good DATA cable, NOT a phone charging cable. Power on the RC AFTER connecting it to the PC for it to be detected properly.
• 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
• Using DUMLdore
  • LOAD: Will load a firmware file for flashing. 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. After pointing to the correct dji system.bin file, click on flash.
  • FLASH: Uploads the firmware file in the Load step to the device and begins the upgrade/downgrade. Follow the prompts and DO NOT RUN ASSISTANT until you get the prompt that it is OK to do so.
  • 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.
  • UNLOCK: Will unlocked a grounded Spark OR Mavic due to not performing the firmware update.
  • ACTIVATE MAVIC: Offline activation of Mavic

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)