Question about Intel Core i5-8600K

So if you want to replace the CPU, then at least the mainboard and the RAM must also be replaced (possibly the power supply unit). A CPU cooler is also required for this CPU (not included in the scope of delivery).

Sorry but I have to vehemently disagree with Brimstone. You can upgrade the HP ProDesk 400 G2 MT with an i7 4770 without any problems (as this configuration has already been available from HP). Just make sure that you set the case fan in the bios to level 2 - 3 because this CPU has a 24 Watt higher TDP and gets correspondingly warmer. So forget about the mainboard etc. Replace.