You have to undress the leather off the seat back rest.
Unzipping the long zipper running all across the bottom side of the seat backrest does that.
You then fold the leather cover up, as you would undress a T-Shirt (that is here and there glued to the foam backing).
Once you get it up over the headrest at the back of the two bars holding the headrest in the black plastic where the bars go in locate two small holes (one on each side).
Insert a pointy tool (like the smallest Allen keys you have in your "L" shaped Allen key set, but two long nails or any kind of makeshift tool will work too) and simultaneously press in to release the headrest.