Comments are not "first class" citizens of the site – these are the questions and answers. Everything should still make sense when all the comments are removed, so please put all relevant information in the question (if it could be considered part of the question) or in an answer (if it belongs there). Both types of posts have an edit button, available for anyone.
You should normally be able to delete your own comments – if not, check if you are actually logged in.
Editing is limited to some short time period (except for moderators).
If you come around some comments which are obsolete, please flag them as such. We'll normally delete them (or in rare cases edit them).
Sometimes I go through old questions and delete lots of old and now irrelevant comment threads, even without such flags.