Sorry but they are incompatible.. I had my doubts and searched for an answer to be sure.. it turns out they use the same physical socket but they have different electrical pinout. In any case the advantage is only 37% better performance but more than double the price. A Kaby Lake i5 would be your best choice for upgrade. https://ark.intel.com/products/codename/82879/Kaby-Lake
"Although desktop Coffee Lake processors use the same physical LGA 1151 socket as Skylake and Kaby Lake, the pinout is electrically incompatible with these older processors and motherboards."