PB:Enabling and disabling I/O
Over at Adafruit is a great introduction to the Device Tree, that is used on BeagleBones.
BeagleBone black originally came with a kernel 3.8 which had a Device Tree Overlay (DTO) and a capemanager. But in the newer kernels, at time of writing 4.9, the DTO has been completely reorganised and there is no need for at capemanager any more.
For kernels newer or equal to 4.9 follow the guideline below.