Suppose there is a sketch and you have added Fit Spline relation. Later you want to get rid of Fit Spline relation how to do that? Because only deleting the Fit Spline relation does not work.

This screen shot shows I have deleted the Fit Spline relation but still sketch is a Fit Spline.

*****this was in moderation for about 24 hours, so it showed up late ****

I suppose what you really want to do is edit the spline like a regular sketched spline.

First you delete the Fit Spline relation as you have done.

Next you RMB on the spline and select Convert to Spline. This will add control points to the spline, and you can edit it like a normal spline.

You can't go back to the fit spline, however. The best you can do is assign tangency and make the spline points coincident to a sketch or edge.

You can edit the points and move it around. If the spline gets wonky, you can select the spline and click Relax Spline in the PropertyManager. The Proportional switch can sometimes also prevent you from editing the spline the way you want to.