You can mark a message as unread immediately after reading it or from the Inbox page. By marking a message as unread, the message details will appear in bold.
At the top of your dashboard, click the message icon to open the Inbox page.
Click the message you want to read.
After reading the message, on the top right, click the Mark as unread button.
From the Inbox page, select the checkbox for the message you want to mark as unread.
Select the Mark as unread option from the drop down.
For more information, watch this-