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:

  1. Open Avast Security Premium
  2. Go to Menu and select "Enter activation code"

  1. Enter the product key you received after purchase and click Enter

  1. Now click on "Activate and Install"

  1. Done, you have successfully activated your product