I can't edit my own comments on my own questions and answers. I can still delete and add a comment, so if I really wanted to, I could copy my previous comment, add a comment, paste it in, make changes, and delete the old comment. This happens on all communities that I am on, regardless of the community. One example:
But there is no edit button. This is my own comment on an answer to my question. On another community:
Alas, no edit button, even though there is a delete button. I could copy the comment, delete the comment, add a comment, paste it in, and make changes. As of writing, I have < 50 reputation on all communities. If it is not currently possible to edit comments, please add the feature-request tag to this question, instead of down-voting, deleting or closing this question.