Stop free trial service "Avast Internet Security"

Hi there. I am writing cause I started a free 60 days trial service “Avast Internet Security” and now i want to stop this contract and come back to use “Avast Free Antivirus” before Avast Internet Security ask me payment but i don’t know how to do it. Do you have any suggestions in matters?
Thanks for your attention

Giuseppe

https://www.avast.com/nl-nl/faq.php?article=AVKB12