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