We're not sure why Tesla is reinventing the headrest on the Model X, but for some reason the automaker decided to make it so that the headrest only adjusts automatically based on seat location. No manual control of the headrest is available at this point in time.
As video uploader Bjorn Nyland states:
"One of the new features of Model X is automatic adjustable headrests. It will go up or down depending on seat position. While this is not the best solution, it is still a step towards the right direction. Maybe a software update can allow the user to set it manually."
"My wife is 151 cm and prefer having the seat all the way in the back for better legroom. So in her case, the headrest will go all the way up which is not optimal."