To Modify a B-spline Curve Using Handlebar Editing

Select the Curve Handlebar tool ( Modeling > Curves > Modify Curves ).
Select the B-spline curve.
The B-spline is highlighted and a point displays on the curve, controlled by the pointer.
Enter a data point to locate the point on the curve to be modified.
Do one of the following:

Click and drag the outer handles to alter the tangent direction of the curve.

or
Click and drag the center handle to modify the shape of the curve.
(Optional) Click another part of the curve to modify it.
Enter or Reset to finish.

Modifying a B-Spline Curve Using Handlebar Editing