Canvas does not allow you to delete student submissions for student annotation type assignment. However, students can re-submit the completed annotations if you have enabled unlimited attempts by setting the “Submission Attempts” to unlimited. This will allow students to re-submit the assignment as a new attempt.
By default, Canvas will display the latest attempt in SpeedGrader. You can select (and download) the files from the earlier attempt(s) in Speedgrader by selecting the relevant attempt from the “Submission to view” dropdown.