OK it is explained all over the internet, even your mom know it is a bad idea, but still you want to try and see if you can tweak 5-10% extra power, I tried to go from 1200 standard speed to 1300, that is +8% ya ya it worked, so sysinfo (Amiga speed test program) said i went from 7.5 to 8.0 times Amiga 4000/40 speed, but can you feel the difference ? NOT AT ALL.. ok lets try 1400MHz +16%, and crash !! system refused to reboot, the hold shift trick to disable overclocking did not work, so i changed the SD card to another older non overclocked version just to see the PI 3 was still alive, ok it was, while booted up with this older card, the overclocked SD card was put into a USB reader, and config.txt was modified back to original values, but still the pi refused to boot with this SD card, its data was corrupted. The only way to fix it was clean format, and image restore. So days of program install and settings many places was lost, for nothing..
This post is to make sure you dont waste your time, if you need more speed, the hardware you are using is wrong brand.