If you accidentally read a conversation, or if you just wanted to pay more attention to it later, in the Inbox tab you can mark a specific chat as unseen.
Simply select one chat you'd like to mark as unread. The chat will change color, and you will know to which people you still have to write back. Then, you can filter your inbox and see the unread chats only or simply all the messages.
However, this option will only be visible in your Expandi account. This will not affect the LinkedIn conversations. So once the person noticed that their message was already seen, it cannot be changed using this feature.