I keep seeing the “Buy Now” button though I am already own TDB

Until you successfully activate The Discovery Bible software with your personal user identification and license key, you will see a message about your "Free Trial" or Purchase each time you open the software. You do not need to download the software again. Simply complete the activation process as explained here so the software will continue working on your computer and you will no longer see the "Buy Now" message or get restricted after the free trial period.