If you upgrade the tuner to crom 9r28 you wont have to downgrade to AC as 68052005AD is now supported. If you still want to downgrade, after plugging the tuner into the car and agreeing to downgrade it will tell you to hook up the tuner to your pc and go to www.diablosport.com/dcx. (This is from memory so may not be exact.) Once attached to your pc, run the DSdownloader and you will see a tab that says update calibration. Click the update button and it will put the AC calid onto the tuner. Now you can attach it back into the car and downgrade. Essentially you dont download the AC calid from the website, but it is done through dsdownloader.