To install the app: sudo apt install bm818-tools
(for some reason the package name is in the plural, but the app label, once installed, is in the singular: (“BM818 Tool.”)
To upgrade the modem’s firmware to the September 2022 version, Support has said to contact them for instructions, but as they’re so busy lately… Someone (@lakei ) posted the instructions here recently. (I just checked and verified that the package names to be downloaded from Purism’s site are still the same.) [EDIT: Don’t do this without first contacting Purism Support for correct/latest instructions.]