Hi Debbie. Upon checking the manual, there seems to be no option for reminder for unread text messages. There are apps from the Play Store you can use for this purpose.
Prof Reminder - missed call - The application will notify you of missed calls and unread SMS / MMS.
GO SMS Pro - This is a messaging app. It's a good replacement for the stock messaging app. It has a lot of features including reminder for unread text messages.
Let me know if any of these two apps works for you.