Thickness of a NURBS Tube

Support Modeling
1

How do I modify the thickness of the NURBS Tube?

2

I don’t think you can, not directly.
But you can convert to mesh (Alt+C), then run the Solidify script (Mesh->scripts->solidify selection) on it. Or - if using a newer version of Blender 2.50 - the solidify Modifier.

Good luck.