Secondary cameras on the top of the helmet, with smaller displays above the main overlay? So when the wearer tilts their head far enough back the internal display removes the stuff at the top of the HUD and replaces it with what the higher cameras see.
I.e. a normal usage HUD might be:
- Top row of data (i.e. compass, armor status)
- Main data (i.e. enemies, ranges, terrain in front)
- Bottom row of data (information about the ground you are stepping on, ammo count?)
But when the wearer tilts their head back far enough, the top and middle of the display swap:
- Main data (i.e. enemies, ranges, terrain in front)
- Top row of data (i.e. compass, armor status)
- Bottom row of data (information about the ground you are stepping on, ammo count?)
In the case a Battle Armor's head won't rotate back far enough, the internal layout of the helmet is where the change is made. Of course the wearer's head still has to be protected/padded frfom impacts so that will be a fun bit of engineering.
A second option is a relatively cheap suit that the wearer can turn their head left and right, move their eyes up and down, but has to roll onto their side in order to look up (i.e. they lie on their left side and turn their head to look to the right).
A third option is they have a video feed from their gun or some other sensor and use that to look with.
Of course this can vary from design to design. Essentially the less armor there is protecting the neck of the Battlearmor, the more flexible it will be and the more likely the wearer can just look up. Aka whatever TPTB or the writer decides.