To manually change the shading requires a full modeling suite, like Blender, or 3DS Max.
I don't use blender, so I'm not clear on how exactly you modify the Shading Groups in that software, but in max, I can show you.
I'll use one of my models as an example here...
This is a model I put together in Silo, and Imported into Max. Now, because Silo is a generalized, bare-bones modeler, all it does is give you a mesh with a generic shader. A really ugly one at that.
Keep in mind these steps are probably specific to 3DS Max only, but I'm showing this more to demonstrate how to modify smoothing groups rather than actually providing a full tutorial.
The next step is to select the object in "Element" mode, so that the entire model is highlighted...
From there, you look over to the side panel, and look for the "Polygon: Smoothing Groups" section, and you will see a numerical grid that ends with 32. Plus a slew of other buttons, that is the Smoothing Group control panel with which you can modify the smoothing groups on your model.
Now some people may say, "NO DON'T USE AUTOSMOOTH!! YOU TOOL!!!"
But I'm lazy, and I use autosmooth, because really, it actually gets good results if you use it right.
The key here, is in understanding how it works. Looking at the screenshot above ^^^, you may note that to the right of the "Auto Smooth" button, is a ticker with the setting "45" on it. That is the angle at which light, bouncing onto the surface of the object, will actually wrap around a surface. If set 180, light will wrap around for a full 180 degree around your model. Silo, I'm pretty sure, automatically lights to that setting, which is god awful.
To fix that, you use the angle dial that is used by the autosmooth button. By default it's set to 45. But I personally like having it set to 30.
So I'm gonna do that.
Alright, you may notice a few things. First, after pushing the button, we went from a yellow hightlighted "1" to the gridspaces 1-7 being blanked out. This is good, that means autosmooth did it's job. But wait, the model still looks ****ed up. What the hell?
Well, unfortunately, nothing in 3DS Max is simple, which is why I will forever tell Autodesk that they can suck my dick.
To actually see the results of the autosmooth, we need to do one more thing...
Thats right, you need to scroll up to the "Edit Geometry" Panel, and press "Detach". When you press Detach, window will appear, asking you if you want to detach it as an element, a clone, or as a separate object.
We want to detach it as a separate object, to do that, uncheck all the boxes, and hit "OK".
And here is our result. Now, it's not perfect, and it will still require some manual editing, but doesn't it look a ton better than before? And that my friends is why Smoothing Groups are a nice thing to have.