There isn't a way for students or instructors to clear answers to an activity and "start over from scratch." However, this isn't a problem!
As long your instructor hasn't closed the assignment or the due date hasn't passed, any changes you make are saved automatically, and your instructor will always see the most recently saved version of your answers. If you want to start over from scratch, simply delete all of your answers manually.
You will not be able to "reset":
Locked sections, unless your instructor unlocks them
Autograded questions where you have run out of attempts
When grading a student's response, click the three dots next to Save Progress and select Reset Student Response. Note: This is permanent and cannot be undone.