Once a user clicks on a new conversation and scrolls to the bottom, it is automatically marked as read. The conversation will appear as read for all other users with access to that contact. If you want to change the read status you can do so by clicking the green check mark in the top right corner above the conversation.
Here we see a conversation that has been read. Once we click the green check box, the icon will switch back to three dots.
We have now marked the conversation as unread. We can see the orange dot and orange highlight appear in the middle panel for this contact's conversation.
In addition, multiple conversations can be marked as unread at the same time. Within the conversation panel, check the boxes next to the messages you want to mark as unread and use the dropdown menu at the top to perform a bulk action.