By default, del is the Delete key and the combination of fn + del is the Insert key. Hold fn + I + D for 6 seconds to switch the del and insert functions. If the functions are switched, press del to insert and press fn + del to delete. Hold fn + I + D for 6 seconds again to switch the functions back to the default.
Additional information here: https://www.keychron.com/pages/firmware-for-keychron-k4