I bought the stellarmate OS and installed it on my 3B+. I did the software updater, but the stellarmate tools still shows version 1.3.5. How can I update the stellarmate OS? Do I have to downloaded it and flash the SD card again?

With the new stellarmate app for android it's asking for serial# and license # which according to the write up on the play store should be found under stellarmate --> Support -->Licenses , but that file directory doesn't exist when you have just the stellarmate OS. I tried signing in with the username that i set up when I purchased the OS, but just get "network error".

Same here, I just get Network Error if I'm on my home wifi or on StellarMate wifi.

Yes, the updater just updates all the software on the SM, to update SM itself to 1.4 you have to download the latest image and burn to SD card, it’s a pain I know... but needs to be done.
Most of the time if your system is all ok, you only really need to use the updater..

Yes, there is a bug with this that Jasem is working on..

So are you saying that in order to upgrade to SM OS 1.4, we have to essentially start from scratch - rebuilding hardware profiles, and re-configuring the system again?

If you run the Kstars and EKOS on the rpi then yes, as they will be gone, but I only use SM as an indi server and run Kstars and EKOS from a seperate laptop wirelessly, so not much of a problem for me, I think many people do it this way.... sorry but there is no way to update the SM OS any other way...