I hate that you can't edit your caption but if you go to your photo and click "comment" you can delete the caption you wrote and make a new one by commenting. If someone already commented on it, the new caption/comment wont be right under the picture, but thats your best option if you notice you want to change it right away. To delete the comment click comment like I said before then put your finger on your picture and swipe to the right. You'll see a little trash can that will delete the comment and then just add your new caption! That way you don't need to delete the entire picture and start from scratch. Hope this helps!