Assuming that you are referring to the card comments/rating system then you can delete an old comment and provide a new one. Obviously if you just want to make a simple change you should copy old text and either paste it into the new one or, perhaps better still, into a text editor rather than type it all in again. Not as convenient as directly editing a comment but not very difficult.
You can find the delete option if you go to the Edit Profile tab and get the list of all your previous comments - each is accompanied by a pull down menu offering a Delete option. I only found it recently after several years wishing that I could edit a comment and then only after I had been told by another user where it was hidden.
If, on the other hand, you are referring to editing comments made as posts to the forum then you can edit them as long as there have been no latter posts to the same thread.