I currently trying to get some help from Megarevo regarding the firmware update and am going around in circles, but I did find out that there is a different Solarman website that has a "fleet" view of inverters and there is a firmware update option on that.
Go to
https://pro.solarmanpv.com and create an account. You can then add in your stick-logger serial number and it will show up on the portal. I've attached some screenshots. Note: the numbers look weird because my inverter is in "stuck" mode (this is why I'd like to update the firmware). It is running properly but not updating metrics.
Unfortunately, the "select firmware update" option is currently empty and I am trying to work through support to understand why.
Note: here is a YouTube video of an upgrade (they do the stick-logger firmware, but you can see Deye firmware in the list):