Modify panel > Select NURBS object. > Stack display > Curve sub-object level > Select an independent CV curve sub-object. > CV Curve rollout > Rebuild button
The Rebuild button for CV curves displays this dialog. It lets you specify how to rebuild the curve. Rebuilding the curve can change its appearance.
Tolerance—Rebuilds the curve according to accuracy. The lower the Tolerance value, the more accurate the rebuild. Increasing Tolerance enables the curve to be rebuilt using fewer CVs.
Number—(The default.) Lets you alter the number of CVs in the curve.
Preview—When on (the default), your changes are previewed in viewports.