Hi Meng,
It is not possible to change the FOV / Focal Length of the VR Camera. One of the reasons for this is that it would induce nausea. (Also a specific barrel-distortion is applied to the image AND the view is calibrated to a specific focal distance)
I think the best path forward would be to look into using Render targets, (Basically a Camera that is feeding view to a texture, then you’d change the FOV on that camera)
Good luck!