Kindly have a look at the Bitdefender Community Forum link provided below, and the answer given by Bitdefender Community Forum admin @Alexandru_BD in the same thread.
https://community.bitdefender.com/en/discussion/90147/how-do-i-change-email-that-was-used-when-bitdefender-was-started/p1?tab=all
Regards
Right here:
https://www.bitdefender.com/consumer/support/answer/13002/
Should you encounter any difficulties in the process, you can reach Bitdefender Support at the link below:
https://www.bitdefender.com/consumer/support/help/
And they can help you out with this.