There are snippets of information available.
Here is a document link from Quectel on how to download and install firmware update over the air (DFOTA). It is applicable to the EC21 modem.
Quectel Procedure Document Link
Unfortunately, Quectel themselves do not provide an FTP/HTTP server on which to host the file. They just send it to you via Email. And my problems is that the darned modem does not work properly anyway, so getting it to go online via 4G is a distant goal, not an established possibility. It is WHY I need to do the update.