The real question is tricky. I have to do a simulation of air flow in intake manifold and want to change the diameter of inlet and outlet. But my model does not seems to change a bit from traditional methods. Looks like catia doesn't want me to edit it. I will attach the model. Change any one diameter of the circular face and my concern will become more clear.
I didn't download or open your model, so my response is based on the picture from a previous post. I'm also assuming the solid is a "dumb solid" with no history for proper editing.
In the part design workbench, you can use the Thickness command to add or remove material to the solid shape. (+ thickness will add, - thickness will remove)
This is not the best way to modify a part, but since it's for analysis only, it should work.
It has nothing to do with catia, rather it is the nature of 3d models output to formats such as step and iges to lose the parametric features and thus their adjustability. Some CAD programs can attempt to recreate features on import, but I haven't yet seen it work well on any part of significant complexity.
Your best bet, if you wish to iterate a design, is to create a well behaved model that fully captures your design intent and allows you to make intuitive adjustments to that intent.
In fact, my recommendation with parts like manifolds, pumps, air movers, and such is to actually model the air first! That is because it is really the shape of the air (or fluid, or gas) that you wish to control, therefore that shape, or path, is the true intent of the design.
The 'solid' geometry surrounding the air, such as the material that the manifold is made from, is very simple to construct and drive right off the 'base' model of the air.
This will allow you to do your CFD analysis earlier, and generate actual part geometry later, which is more efficient, and the order it wants to happen in anyway, project schedule wise.
It's weird, and counter-intuitive, but oftentimes it's so much easier to model the inverse and then derive the actual part from that.
Maybe the following will help you understand what a STEP file is: