I wish to upgrade from Cubase Elements 6 to Cubase Pro 6. Is that possible, or do I have to buy the newest version of Cubase? Cubase Pro 8.5
This is not available from Steinberg. They only sell the latest software versions. But, you might be able to find something from a private seller (ebay?). If you do purchase from a private seller make sure that you/they follow the instructions as mentioned in the resale wizard and you get the USB E license key fob. BTW… It’s probably not named Cubase Pro 6. Maybe Cubase 6 Full?
Regards 
You’re very unlikely to find anyone selling Cubase 6 because nearly everyone upgraded to at least Cubase 6.5, but most will have newer versions.