Out-Dated App Promo Code

Hello,

First, I am new to mac so you will have to excuse me ignorance...

Now, I purchased an app that was the first design and the creator then created a newer updated app with new features.

So, I contacted the creator and he sent me a promo code for the newer app... well... I don't want to purchase the newer app

I want to enter the promo code but if I click on install and enter my security info it will then bill my credit card. So... how and

where do I get to enter this promo code. I have looked all over and can't seem to find where to enter the promo code.

Sincerely,

Damian