See [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) then the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information.
STM32F042xx chips does not allow jumping to bootloader without BOOT0 being set to high, therefore it is impossible to enter the bootloader from sending a `RESET` keycode nor using bootmagic or bootmagic lite.
The only way to enter bootloader is to hold the BOOT0 button while the keyboard is powering up or after a power reset (done by pressing the reset switch or sending a `RESET` keycode).