That's correct, @KevinRM ...
There currently isn't a way to edit comments you leave for submitted assignments on the right-hand panel of the SpeedGrader. In looking at this Guide, How do I leave feedback comments for student submi... - Instructure Community - 764 (canvaslms.com), there doesn't appear to be a way to edit existing comments that have already been submitted back to the student. You'd have to post a new comment or delete the old comment to post the new feedback with the correction (you could just copy and paste the old comment, make the edits, and then submit the comment again). Keep in mind, if you do this, then a new date/time stamp will show for your submitted comment back to the student.
I have some good news for you, though! I just searched the Community to see if there were any existing Ideas that had been submitted about this same topic. I came across this one, and it is showing a status of "In Development"! [Speedgrader] Editing Comments in SpeedGrader - Instructure Community - 579582 (canvaslms.com) Now, I don't know exactly when we'll see this released, but I hope it will be of some help to you. Consider following the Canvas Product Roadmap - Instructure Community (canvaslms.com) so you have an idea of what Instructure is working on for the near future.
Hope this helps.
