Wiring question on replacing a 240 volt wall heater with a 120 volt heater
It looks like you have red , black, white &amp; ground. You could connect a 120v unit to the black , white, and ground with the black, white and ground of the heater. This should be 120v if properly wired. The question would be  is this the only heater on this circuit? Also what is the wattage of the new heater. A 120v heater will draw 2x the amperage of the same sized 240v unit and the wiring may not be large enough. If the new heater is 1/2 the wattage of the old one using the black or red wire for the line voltage and then white for neutral and bare copper to the ground will work.