You have to download and install Avast Premium Security from here: https://www.avast.com/en-ca/download-thank-you.php?product=PRW-ONLINE&direct=1
After downloading and installing Avast Premium Security:
- Open Avast Security Premium
- Go to Menu and select "Enter activation code"

- Enter the product key you received after purchase and click Enter

- Now click on "Activate and Install"

- Done, you have successfully activated your product