Here are some common issues with Godox firmware updating:

Mismatching firmware file

Whatever you do, DO NOT USE A FIRMWARE FILE FOR A DIFFERENT DEVICE. This is a basic tenet of all firmware updates: firmware is closely tied to hardware configurations, and any hardware differences can mean bricking a device if you overwrite with the wrong firmware image.

With Godox speedlights, the E9 error means you've loaded the wrong firmware image. You might be able to fix it if you can overwrite it with the correct one, but you may also have a bricked device that needs to be returned.

While a TT685 and V860II might be identical in UI and features, the fact that they both use different power sources means the firmware images are not identical. Each device has its own separate firmware file for a reason. TT350 users found this one out the hard way when Adorama mistakenly posted the TT350-C update (specifically for adding compatibility with some newer EOS M bodies) as a generic TT350 update. So many non-Canon TT350 owners bricked their flashes with this update, Godox eventually posted the initial versions of the TT350 firmware.

Similarly, there are several bricked X2T-C/N/S transmitters at this time, because there are "Z01" hardware versions [check your battery covery] as well as non-Z01 hardware that use different firmware images.

If your device does not yet have a firmware update posted, suck it up and wait until it does.

The only way to fix a mismatch like this is to reload the proper firmware image, but this depends on the device being able to recognize the firmware update request, and sometimes the correct/current firmware image you need is not available. Requesting the firmware from your retailer or Godox may be your only recourse.

G1, G2, or G3

Godox has three separate firmware updating apps. Which one you want to use depends on which devices you have and when they were released chronologically by Godox.

G1 G2 G3 V860
V860 II
TT685
X1T/X1R (C/N/S)
AD360 II
AD600
AD600M AD600 Pro
AD200
X1T (F/O)
TT350 (C/N/S/O/F)
V350
XPro (C/N/S/O/F)
A1 AD100/200/300/400/1200 Pro
V1
TT350-P
X2T
XPro-P
P2400
TT685 II
V860 III
XProIIL
[anything newer]

G1 is for the oldest devices that use .fri files; the G2 is for AD600 Pro and later (but pre-AD400 Pro) devices that use .dfu files; G3 is for AD400 Pro and later devices that use .bin files. Check the Godox downloads page to make sure you're using the correct application for your device. G1 and G2 installers will also launch a second installer to add a custom USB driver.

Bad USB cable

If the Godox application doesn't recognize the device, consider trying a different cable. Some USB cables are solely for charging and do not have the connections for data transfer. And, of course, your cable might just be bad.

There are also reports that for the units that have a USB-C port, that you need to use a USB-C to USB-A cable.

Wrong USB port

Make sure you're using the correct cable and port for firmware updating the device. This is not as easy as it sounds for the AD Witstro lights, which often come equipped with two different types of USB ports.

The USB-A port is only for attaching an FTR16 receiver if you want to use Godox's older 433 MHz triggering system. Always use the USB C (newer lights) or micro USB (older lights) port for firmware updates.

File names

If you downloaded your files from the Godox site, rather than the Flashpoint one, pathnames of either the updates or the applications may contain two-byte Chinese characters which can cause an error. Removing the two-byte characters fixes the issue.

The downloaded file is a .rar archive that contains the firmware update file, and a PDF that contains the revision history for the firmware. If Internet Explorer renamed it as a .man archive and you do not have tools to extract the archive contents, you can simply rename it to .rar.

Also, the .rar format has changed in recent updates so you may simply need to update your archiving application (e.g., update Winzip to the latest version). Or you can use the latest version of the open source 7Zip.

Disable driver signing [Windows 10, G1]

FluidKnowledge on dpreview mentions that you may also need to disable driver signing in Windows 10 to get the G1 updater and driver to work properly, as it's not signed (being written for Windows 7).

The simplest steps to do this (at time of writing) are:

1. Hold down Shift while selecting [Start menu]→Power→Restart. Continue holding down the shift key through the spinning circle of white dots until you are in the blue advanced boot options window.
2. Select Troubleshoot.
3. Select Advanced options.
4. Select Start-up Settings. You'll see a list of options, and a Restart button.
5. Select Restart. More spinning circle before you can select how you want to modify the bootup from the list of options.
6. Type 7; to select "7) Disable driver signature enforcement".

You box will now boot up with driver signing disabled and Secure Boot turned off, and G1 should now be able to recognize your Godox device over USB. Once you restart the box, both driver signing and Secure Boot are re-enabled.

G2 looks like it uses a separate DFU driver(?) that apparently is properly signed for Windows 10. G3 does not use an installer and does not add a driver to the system.

Taking out the batteries

Some folks are reporting that you have to remove the batteries from speedlights or transmitter before attaching them to USB to get the firmware updater to recognize it.

See also: Flash Havoc's article on the G1 app.