The short answer is "no".
The comments from the Conversation tool are transmitted by text message and/or email, and so, they are not editable. Comments posted to the Conversation are a historical view of what was said, either by you or the student. All messages are retained in their original version. This helps to preserve exactly what was communicated.
Should you make a mistake and wish to clear up any potential confusion within a message, we recommend simply posting another message which your student will receive via their notifications preferences.