OK, how do we do that in 2019 now?
It would probably have been better to create your own new topic, as much has changed, rather than resurrect a very, very old topic.
You don't say what it is that you want to switch back from ?
If is is Avast Internet Security trial, this isn't too difficult go to Control Panel, Programs & Features, Uninstall Avast Antivirus, this should give the option to go back to Avast Free.
However, if this is a 60 day Trial version ?
Much has change as you would have needed to give payment instructions so payment can be made at the end of the trial. This mandate would also need to be cancelled or payment would be made, even though went back to Avast free.