I wanna go back to free plan after the license expires (in 20 days) but can’t find it anywhere.
Just delete avast and it will give you the option to revert to the free version.
For trials > https://www.avast.com/faq.php?article=AVKB12
You can always take the long way. 
- Remove avast completely
- Install the latest free version
It’s much easier to simply uninstall Avast and accept the offer to go to the free version. 