How to modify the film aspect ratio of safe frame in 3dview

I don’t know how to modify the film aspect ratio of safe frame in 3dview. I see that the default value maybe 1.78 for 1080p. But in most cases, I will not use 1.78 to see the composition in 3dview.
I want to know how to change the film aspect ratio of the camera to 2.35 or other values.
I want to know how to change the film aspect ratio of the camera to 2.35 or other values.