It is currently not possible to edit a comment. If comment editing was possible, users could make a rude comment and quickly change it back, meaning it would be harder to report them. Though enabling comment editing would allow users to fix mistakes, it is still disabled due to the easy workaround of replying to themselves, saying they made a spelling error. There have been suggestions to add this feature. However, it has been rejected. However, it is possible to edit posts on the Discussion Forums after posting.
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