As of right now, there isn't a way for Posters or Helpers to change or remove a comment. This means the compulsory comment that has to be included with an offer on a task is the sole exception to this rule. 

The offer's attached comment will be deleted when you withdraw it. The choice is then yours to remove your previous offer and replace it with a new one with a new comment. This feature is helpful since you can notify the Poster if your circumstances change.

Try contacting us if you'd want to have one of your comments deleted because it no longer applies or you've learned it doesn't follow the Community Standards.