Instagram updated the app and made some changes regarding comments. They gave users few new features now.
Under each post, after you start getting a lot of comments, there is new, pin feature. You can pin up to 3 comments you choose, to the top of your post. You can do that with swiping left and tapping the icon.
Tap to manage comments
Another new feature is under 3 dots at right top on post view. Just tap 3 dots and you will manage comments or change who can comment on post.