thank you !
Yes. Not sure if the trial of Premium will automatically revert to the free version after the trial period is over, but even if it does, I would still recommend a complete uninstall of the Premium version followed by the install of the free version.
thanks a lot for replying gandalf 
It depends where you have started the trial from.
If you took up the option from within the existing free version to do the 20 day trial there should be an option, see #### below.
If you installed a normal version of the avast Premium, then you can use that for 30 days unrestricted (trial) at which point you would have to purchase a license or uninstall it. Then install the free version.
- Revert from Trial Version to avast free - From the avastUI > Maintenance > Registration, scroll down to Other Options and the “Switch me back to avast! free” option is there.
thank you david !! but i cant find that 20 day trial from avast 8 ui ??
Well you should know how you actually installed it, that determines if it can be a button click to revert. If that isn’t available from the the avastUI > Maintenance > Registration, scroll down to Other Options and the “Switch me back to avast! free” (as in the image, click to expand).
If not then your only option is to reinstall avast free.