Your browser does not seem to support JavaScript. As a result, your viewing experience will be diminished, and you have been placed in read-only mode.
Please download a browser that supports JavaScript, or enable it if it's disabled (i.e. NoScript).
What have you tried? This is possible to do via a plugin.
This can also be done by using Mark as unread for all option in the topic tools dropdown, it will mark the topic unread for everyone without making a new reply to it.
Mark as unread for all