To mark a single message as read, tap the message, or tap under the message, then tap Mark Read. The blue unread indicator disappears.
To mark all messages as read, tap the unread message count in the tile bar; tap in the action bar to display the actions menu, then tap Mark All Read; or, if you have the navigation for unread messages turned on, you can tap the unread message count to mark all read.
Document annotation messages are marked as read when you view the annotation in the document.
To mark a message as unread, tap under the message, then tap Mark Unread. The blue unread indicator reappears.