How to untick and redo a course

I need to start a freecodecamp course from beginning and undo previous challenges in that course. is it possible ? if it’s possible guide me to do it

You can reset your account in the settings, bringing everything to undone.
You can’t untick the challenges of a single certification.

You can anyway go back to the first challenge of a certification and redo all of them as many times as you want.