The HUD is projected into 'infinity' and there is no way to 'tilt' the HUD image in real life or in the simulation. The only thing that can be done is move the surrounding frame. Which aircraft are you flying?
(Since the 1978 San Diego crash where a 727 ran down a Cessna because the 727 crew had their seats too low and couldn't see the straight ahead view (with a murky horizon) it has been mandatory to properly use the seat height view reference indicators. There should be only one HUD viewing angle).
I had my virtual seat too far back with my view excessively zoomed to compensate which narrowed my HUD display width.