As we got so many requests from Debian 9 Users that wanted to upgrade their installation of Plasma 5.8 to Plasma 5.12 we created an upgrade script that will take care of importing our kde-repo into the system and upgrading the desktop to Plasma 5.12. All you need to do is download the script make it executable and execute it as root. (If you want to know what the script is doing take a look at its sourcecode first) 

You can get this script from here

SHA256Sum: 13674571875a96dcb56a4fba2f1fb46a6c929e5223dde8006e68ed9082bffa94