Cannot downgrade to free edition

Chee, Are you using the 30 day trial version?.
AVG Guru

I have AVG Security 2016 installed on my notebook, however I can't downgrade it to free version. Please help me to solve this.

Yes. Now I'm using the Remover to unistall and reintall it but I have 0xE001F90F error occured

Chee, Have a look @ this article (https://support.avg.com/SupportArticleView?l=en_US&urlname=Download-and-install-AVG-AntiVirus-Free).
AVG Guru