I created a Multiple layer PCB. I am not able to edit the Zo value of plane. Even If I change the Z0 value to zero, it automatically changes to 75 ohm.
In general, Z0 is zero for ground and power plane.
You cannot edit the Z0 of a plane. And also it's not correct to attempt that also And the auumption of Zero impedance of solid planes is also not correct.
If by any chance you are routing some signals in Power / GND plane and want to analyze them, then define that layer as routing plane in your CAD tool and import the file to HyperLynx. But I am sure you are not trying to do this..