1.  At the top of your dashboard, click the message icon to open the Inbox page. All messages will be displayed.

  2. Select the checkboxes for the message you want to mark as unread or select the checkbox in the header. All messages that are currently visible on the page will get selected.

                                                                   Fig 1.

  1. Select the Mark as unread option from the drop down.

                                                                                Fig 2.             

For more information, watch this-