WE DO NOT RECOMMAND editing comment, except for misstypings.
If you edit a comment, you'll change the history of the work done on this project. Your comment may seam useless at the moment, but turn very usefull later. Your best option is to add a new comment, even it this new one say : « I made mistake in the previous comment, now I feel like it's better to .... than ... as it is said up. »
Anyway, you're the one who will manage your projects, issues and comment. So, if you want to edit a comment do as following: