Two ways to do this: The first is simple, but if your install records marked it as a CC app you may have to go the second route.
Permanent reverts to Trial with CC ad.
This happens often. Don't panic. Your serial number is still good, you simply need to re-enter it and you do not need to purchase anything. Follow these steps:
1. On receiving Trial/Trial Expired screen
2. Make sure that you are online
3. Click on license this software
4. Perpetual product owners: Log in with adobe ID and enter product serial number
5. Product should be licensed successfully
If not:
You may have to uninstall both Photoshop and the Creative Cloud Desktop App, use the Cleaner Tool
Use the Creative Cloud Cleaner Tool to solve installation problems to remove the install records and fresh install CS6 using your original serial number.