Normally subfolders are set to inherit permissions from their parent, this needs to be disabled first.
Select folder and right click -> Properties -> Security (tab) -> Advanced -> Permissions (tab) -> "Change Permissions..." (button)
Remove tick from "Include Inheritable permissions from this object's parent...". You can either keep the permissions, and edit them to your liking, or set completely new ones a this point.
Don't forget to Apply/OK