If you make a mistake with a comment, you can delete or edit it. Teachers can also delete student comments.
To delete a comment, tap the gear icon next to the comment:
Tap "Delete Comment":
The comment will be removed from the conversation:
At the moment is isn't possible to edit comments on iOS, but you can copy the text, delete your old comment, and create a new comment. Tap the gear icon next to the comment:
Tap "Copy Text":
You can delete the old comment as described above.
Then create a new comment, and paste in your text. Edit your text, then add the fixed comment.