Just describing @bignastyid's answer in more detail,
Every motherboard has a specific CPU socket type. Your current motherboard has an LGA1155 CPU Socket and i7 7700K has an LGA1151 Socket, so it simply won't fit. The best CPU you can get for that motherboard is Intel i7 3770K, which is too rare and extremely overpriced these days.
Either start with a completely fresh build with latest gen motherboard and CPUs, or leave it alone. (Until you find a good compatible CPU).