Because of the nature of how submissions behave in Canvas, once they have been submitted, they can no longer be edited without a new submission being created.  If students are to continue to work on an assignment after having made a submission, they would have to create a new attempt, and much like getting a new copy of a completed worksheet to try again, that new attempt will be blank of previous work. 

If you would prefer that students be able to edit the annotations they created then they would need to use a different means of annotating the files (like with a pdf editing software on their device or an external tool) and the assignment allowing file upload submissions as well.  Additionally, if file uploads were allowed, students could download their initial attempt and then edit it using an external tool or software and then submitting the edited work as a file upload instead of an annotation.