I want to modify a curve by changing its edit points or greville points, not by its control points. How would I do it with rhinocommon functions?
I do not believe there is a function to modify a curve with edit points in the SDK. You can easily extract these points and for that you can check NurbsCurve.GrevillePoints Method in http://4.rhino3d.com/5/rhinocommon/
What do you need it for? If you need a curve to go through certain points, can you instead create an interpolate curve?