You must possess editing permissions in the Commons tool in order to edit a comment/reply.
Go to the Commons tool.

Select the Commons tool from the Tool Menu of your site.
Select Edit.

Locate the comment/reply that you would like to edit and select its associated Edit option.
Edit the comment, and then select Post Comment.

- Make changes to the comment in the text editor.
- Select Post Comment.
Verify the edited comment/reply.

View the edited post to verify that changes were made successfully.