Kenny wants to root his new Samsung Galaxy S4 phone to update the OS to Marshmallow. Leo says rooting the phone will give him administrator priviledges, and Samsung discourages it because their security features could get bypassed by a malicious app. If he roots it, he won't be able to use Samsung Pay either.
Once he's rooted it, he'll want to put on a custom recovery mod. Leo likes Cyanogen Mod, and version 13 is based on Marshmallow. To learn the details on how to do this, Kenny can go to the XDA Developers Forum. He should search for the exact model number for the phone he has, and he'll get step by step instructions on how to root and install the mod he wants. He should make sure to follow the instructions to the letter.