Click the Notifications icon (the bell-shaped icon) at the top of kintone.

On the "Notifications" screen, click Read.

Select the Batch Process checkbox at the top right of the screen.

Click the Mark as unread icon (the plus sign-shaped icon) at the top right of the notification that you want to mark as unread.

Click Unread to display your unread notifications and check that the notification you marked as unread is displayed.