Changing your mind about someone used to mean opening their profile, unfollowing, going back, and finding your place in the list again. Now every row in your followers and following lists has its own button, right where you're already looking.
The row stays put after you tap it, and the button flips to follow — so an accidental unfollow is one tap away from being undone. Your followers list works the same way in reverse: anyone you haven't followed back gets a follow button, and the change shows up everywhere that person appears.