Since Samsung Galaxy S6 is an Android device it is possible to overclock and underclock it. In this tutorial we will teach you how to overclock and under clock G920F and G920I Galaxy S6 easily using custom kernel. We suggest you to set the range between 200 MHz and 1600MHz as otherwise your phone will become unstable.
First you need to download and install the official xda custom recovery for Galaxy S6 from here. Now that you have installed custom recovery, you should download the G920F and G920I compatible kernel from here. Then place the file into the internal storage and boot into custom recovery by flashing and holding volume up button along with power button. Then select the previously downloaded file and flash it.
In case you own a Tmobile variant, we suggest you follow this post for overclocking and underclocking instructions.