Webmentions are a way to send in your comments and other such likes, no SMS or registration required. If you have a website and it can do it, you know what to do. If you don’t have your own website but would still like to comment, try using CommentParade or another such service.
Thing is, you don’t leave your comment on my site, you do it elsewhere. I get a notification, and a simple server-side app asks the aggregator (webmention.io in my case) for the comments, saves them to my site, and shows them to you. If you feel the need to change or delete a comment, the best way is to change or delete it wherever you’ve published it, and my site will reflect the change (though not immediately, perhaps).