Is there a way to edit instructor's comments on Speedgrader after they've been submitted?
For general comments added on the right sidebar of Canvas it is not possible to edit them. You can delete a comment by clicking the X can icon to the right of the comment. So you could copy the existing comment text, paste it as a new comment, delete the original comment, and then edit and post the new one.
If you are talking about Annotations that you create directly on the submission in Speedgrader, those can be edited. Simply click on the annotation and it should be come editable.
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