If you need to see a preview of the image in square format to help with the composition using the rear LCD screen, it is easiest to suggest it at the end of the question: mask the back LCD in a box, taking care to make the width of the mask equal to Image height of preview images that are displayed on the LCD screen, instead of the height of the LCD screen itself.
For the viewfinder, when you select & # 39; On & # 39; as the option in the custom configuration & # 39; d7: Grid Display Display & # 39; allows you to visualize a set of grid line overlays. The two vertical lines on each side of the frame appear to be about 5% higher than the width between each. This is measured from a drawing of the viewer on page 10 of the Nikon D750 User Manual
I added yellow lines to show where a real square cut would be centered on the exact half of the frame:
That will bring you closer, although you will need to manually crop the image later.
Is there any way to install the D850 firmware on the D750 maybe?
No If you could load it, it will most likely block the camera permanently. Even if it did not permanently damage the camera, it would not work properly. Different pixel dimensions of the sensor could mix the photos. Other hardware differences, both internal and external, would make most things not work.