4k unusable, how to change back to just AUTO HD

For some reason, my xfinity box has begun AUTO choosing 4k channels instead of HD channels the last few weeks. Xfinity and my 4k Samsung tv dont like each other. 4k channels are extremely dark and I can't even read the guide. Is there a way to remove 4k channels or at least go back to where the guide AUTO chooses HD instead of 4k?