You shouldn't. You should replace your i5-6500 with a CPU that is better suited for the GTX 1080.
Games today are getting more dependent on multi-threaded CPUs. Even a GTX 1060 can be bottlenecked in certain games when paired with an i5-6500. So, either buy a used i7-6700K or i7-7700K on eBay; or ditch your current motherboard and CPU and get a Ryzen 5 3600 and motherboard.