How to Edit a Comment

You can't edit an old comment.
You can copy the old comment (use Ctrl/C) and then write a new comment by pasting the old comment into a new Comment window (use Ctrl/V) and editing the new comment before you publish it.
The timestamp will be modified to reflect the time of the new comment.
Then you can delete the old comment. (See the post on "How to Delete a Comment".)