📱 Smartphone

1. Press and hold the chat in the chat overview
2. The options menu opens.
3. Select "Mark as unread"

🖥️  Desktop

1. Hover over the chat and click on the three-dot menu on the right.
2. Select "Mark as unread"

To mark the chat as read again, follow the same steps and select "Mark as read". Alternatively, you can open the chat to mark it as read.