Download the UF2 file linked below. Turn on Hallowing and connect a USB cable to your computer. Double-click Hallowing’s reset button, wait for the HALLOWBOOT drive to appear, then drag the UF2 file to this drive. After a few seconds, the code should be finished transferring and will run.
This will overwrite CircuitPython if it’s currently installed on your board (but your CircuitPython code and any libraries are safe).
You can restore CircuitPython easily by following the directions here.