EdgeTX Companion 2.8 is available for download here (pick the appropriate download for your operating system from Assets heading at the bottom of the page - the filename will have cpn in the name).
If you intend to use mixer scripts, when compiling the firmware for your transmitter you need to make sure the LUA_MIXER option is set. This is not required if you only intend to run any of the other types of Lua script (i.e. telemetry, one-time, function scripts, widgets) as support for those is included by default.