Can I upgrade CPU to I7 7700K?

"Yes! You can replace the CPU in this system. Here's the IFIXIT guide: iMac Intel 21.5"" Retina 4K Display (2017) CPU Replacement.

As to what CPU you can use Apple only used an Intel i7-7700 as the highest in this series so its hard to say if a i7-7700k will work or not.

Even if it does the power load & heat maybe to great depending on what you are doing on the system (heavy intensive processing)

The last issue here is this is such a new system you'll void your warranty and this is still a hard system to open as you need to carefully remove the display which is glued on. Many people have damaged it in the process if you don't follow the guides to the letter and use the correct tools.

Are you sure you want to risk it?"

It's not possible to upgrade or replace the processor (cpu) or the graphics controller (gpu) in an iMac. You can add more memory (RAM) up to the limit of your particular model, and you can replace the hard drive with a larger capacity drive.

Does someone knows if i can make a cpu upgrade to the 2017 21.5 iMac base model? It’s the one that’s only FH and came with the i5 7360U cpu

"Hi,

is it possible to upgrade my 21,5” 4k 2017 iMac with a i7-7700k 4,2GHz?"

https://www.youtube.com/watch?v=knMM9dRw... with i7-7700K