If it's a Late 2015 it does not even work with the standard build of the first El Capitan version and the first betas of 10.11.1. So there is no way to downgrade to Yosemite.
You can only really downgrade back to the original OS your machine came with. There are exceptions to this (but I think skylake stops that on your machine) and you may find some hacks to do it online but there is no easy solution.
You can only really downgrade back to the original OS your machine came with. There are exceptions to this (but I think skylake stops that on your machine) and you may find some hacks to do it online but there is no easy solution.