I currently have an Intel Core i3-6100 CPU and an MSI H110M Gaming, but I want to swap them for an Intel Core i5-9600k CPU and an ASRock 7390 Pro4. Would this be a good trade or should I keep what I already have?
It would be a big improvement over what you have but price to performance I'd take a Ryzen 2600/B450 board over that 9600K/Z390. The Ryzen would be much cheaper and the stock cooler is sufficient not only to keep it cool but also for some moderate overclocking. The 9600K doesn't come with a cooler and you'd need something decent to keep it cool.