Connecting via the app on the phone failed for me as well. 

Recommended solution worked for me with the following steps to upgrade:

1.  Connect my laptop directly to M9 via a ethernet cable

2.  Launch the M9Plus_2.0_upgrade_tool_v1.1.1_win32.exe application (run as admin).  I am running the latest Windows 10 - 20H2.

3.  After it scans all available M9 devices in your network, point to location where the unzipped version of the firmware is located.  In my case it was to M9Plus 2.0_en_1.4.3 Build 20200921 Rel. 42500_up.bin

The upgrade takes a few mins to apply and then it will reboot the device.  

Confirm upgrade version either on the phone app or using a browser via http://tplinkdeco.net.  You should get a prompt for only the owner ID password.

Been having the same problems trying to get the Deco M9 Plus Update Assistant to work on my laptop with Windows 10.

In my case, the login and password is accepted, but when I click "Scan" to identify the Deco units, after about 2 seconds the app simply quits. No trace of it in Task Manager.

What an embarrassment to TP-Link. I really used to believe strongly in this company's product because my past two routers bought from them ran like champs. 

Here is a utility that cannot run on a simple Windows 10 installation without the poor user having to do handstands (try running in virtual machine, try running with older Win 10 version, try running after resetting modem and router in concert, try running it with a direct ethernet connection, try saying a prayer, etc. etc. etc.).

I'm literally too embarrassed for TP-Link to even call their Support team and listen to their lame troubleshooting routine. I'd rather keep trying here on the forums and taking my chances. A shame that a company with a great history is putting out such a crap product today...a router with no Web interface and completely amateurish utility tools. *sigh*