Mark a Notification as Unread
Marking a notification as unread puts it back on your to-do list: it returns to the Unread tab, recovers its unread dot and edge stripe, and raises the bell's count again. Use it when you opened something too early, or when a notification you already read turns out to need action after all.
Steps
Step 1: Open the Notifications panel and find the notification on the Read or All tab.
Step 2: Hover the notification and click the Mark as unread action in its top-right corner. Right-clicking the notification offers the same action.
Step 3: The notification returns to the unread list, and the bell's badge counts it again.
Several at once
To return several notifications at once, tick their checkboxes on the Read tab: selection mode there offers Mark as unread in the header, the counterpart of bulk mark as read. Opening a notification marks it as read again, so use the Received timestamp and the message preview to decide without clicking through. A link inside the message is safe to follow: it opens in a new tab and leaves the notification unread.