If you have a z170 based motherboard, you can overclock.
But, a i7-6700K runs 4.0/4.4 turbo at stock. Really, you do not need to overclock.
It is done by changing bios settings to increase the stock multiplier from 40 to a higher number.
How high is determined by your luck in getting a good chip.
As of 2/16/16
What percent can get an overclock at a somewhat sane 1.40v Vcore.
I7-6700K
4.8 18%
4.7 56%
4.6 87%
4.5 100%