This extension marks topic's last posts and first post as unread if they are edited.
Clone (or download and move) the repository into the folder /ext/phpbbmodders/editedposts_unread:
cd phpBB3
git clone https://github.com/phpbbmodders/editedposts_unread.git ext/phpbbmodders/editedposts_unread/
Go to admin panel -> tab customise -> Manage extensions -> enable Mark Edited Posts Unread