Although is still in development state is stable and has many new features, like simple main screen with large temperature, Fahrenheit mode or left-handed mode.
There is also many features in background
Update is easy and safe with any SWD (ST-Link) programmer
Download from github: https://github.com/pavelrevak/rt_soldering_pen/releases
Updating firmware with cheap Chinese ST-Link programmer:
To program, use ST-Link utility from ST, or any other SW for Flashing STM32 micro-controllers (openocd, texane/stlink, pystlink, ...)
There is also possible to use ST-Link part of any Discovery or Nucleo board.