NURBS Surface Editing
Directly edit the geometry of a NURBS surface by performing various operations such as moving, adding, and removing points; changing the order in the U or V direction; adding weight; and removing trimming curves.

To enter NURBS surface editing mode:
 If the NURBS surface doesn't have construction history, doubleclick it.
 Or if the NURBS surface has construction history, select the object and press Alt+C to collapse the construction history. Select Yes to delete the source objects.

Select the faces to edit:
To Do this Select a face Doubleclick the face. Select a different face While holding down Alt, click a different face. Select multiple faces While holding down Ctrl+Alt, click the faces. Select all faces of the object In the Control Panel, click Select All Faces. 
Edit the NURBS surface:
To Do this Note Delete selected faces In the Control Panel, click Delete Selected Faces. Move a control point  Drag the point.
 Select the point, and then enter X, Y, and Z coordinates.
Add weight to control points Select the points, and then enter a Weight. The default weight factor is 1. The more you increase this value, the greater the pulling effect that the control point exerts on the surface. Weight factor should not be lower than 0.0001. In fact, 0 is not a valid weight factor.
Attention: Weight modification should be used very carefully. If you plan to exchange your projects, you should remember that many CAD systems do not support modified weight values.Modify the knots/control points In the Control Panel, you have the following Knots/Control Points options:  Number of U/V Knots: The number of points along the U/V direction is displayed here. You cannot edit these values. A list of the U and V knots is displayed directly below, with their mathematical values. To select the desired U or V knot, click it in the list.
 Refine by Adding Knots: Select at least two U or V knots in the list, and then click this button. To specify the number of knots to insert between the selected points, enter a value.
 Remove with Tolerance: Remove a selected U or V knot from the list. To specify the maximum allowed deviation from the curve, enter a value.
 Insert U/V Knot at: Enter a value from 0 to 1 to preview where the knot will be inserted along U/V. The preview is represented by a dark green isocurve, over the surface. To actually insert the knot, you need to then click Insert U Knot at or Insert V Knot.
 Display Knots: Knots are displayed in the views as blue curves.
 Reduce Points with Tolerance: To specify the maximum allowed deviation from the curve when removing points, enter a value. Then click the button to reduce the number of points.
 Points (U/V/Total): The number of points along U, the number of points along V, and the total number of points are displayed here. You cannot edit these values.
 U/V Degree: Use to change the surface order along the U/V direction. Enter an integer between 2 and 6. For this option to work, you need to have at least the Degree+1 number of points along the corresponding direction.
Knots are the curves along the surface. Control points are the end points and intersection points of knots.
Remove trimming curves In the Control Panel, you have the following Untrim options:  Interactive Untrim Mode: Click to enter/exit the interactive untrim mode, where you can click the trimming curves to remove.
 Untrim Holes: Remove trimming curves that are holes.
 Untrim Boundaries: Remove trimming curves that are boundaries.
 Untrim All: Remove all trimming curves.
Useful for cleaning up incorrectly trimmed surfaces. Attention: If you have Select All Faces turned on, then untrim operations affect the entire object.  Rightclick and mouse through the check mark to exit, or doublerightclick.
Troubleshooting
Information and recommendations regarding the following error message.
Error Message
A knot using the current value cannot be inserted, as it would result in invalid multiplicity. Change the knot value.
Information
The knot’s multiplicity is the number of times the knot’s value is duplicated. For example, say you have a list of knots with the values 0, 0, 0.2, 0.4, 0.7, 0.7, 0.7, 1, and 1. Knot 0 has a multiplicity of 2, knot 0.2 has a multiplicity of 1, knot 0.7 has a multiplicity of 3, and knot 1 has a multiplicity of 2.
Invalid multiplicity occurs when the multiplicity is greater than the degree of the curve; for example, if you have a degree 1 curve and a knot with a multiplicity of 2.
Recommendations
Change the knot value so that multiplicity is less than the degree of the curve.