Canceling through the Avast website

To cancel your Avast subscription using the browser on your computer, follow the steps which are given below:

Open your web browser and visit Avast Antivirus Official website.
Sign in to Avast using your registered username and password.
Go to Subscriptions
Click on that subscription which do you want to unsubscribe.
Follow the instructions as they appear to complete cancelling your subscription.

Source - Avast Antivirus Refund