I paid my VPN but when I want to upgrade tot Premium, the price did not reduce with the amount I already pay.
ONLY Bitdefender staff can have access to your Bitdefender Central account, your devices, your payments/purchases and/or your subscription(s).
So, you should contact Bitdefender Consumer Support by chat, telephone or e-mail:
Chat is the fastest way to get in touch with Bitdefender Consumer Support.
NOTE: Bitdefender telephone support is not toll-free!
Also, @Alexandru_BD and @Mike_BD (they both work for Bitdefender) can take a look here and help you with the issue. You can PM them.
I'm not sure I understand the situation. You placed a new order to upgrade your VPN to Premium and the discount was not applied ? Did you purchase the VPN from the product page directly, or did you receive an offer via email/product? Did you finalize the order, as I could not find any recent VPN purchase in our records?
Kindly share more information with us, or contact the Support teams for further assistance in this case.