may we ask the reason for downgrading ? - could it be something settled by a compatibility mode setting?
as far as I can see - a downgrade is a complete uninstall and re-install.. but there are other issues. you databases will be set to run on 2017 and if this is anything like when we moved from 2000 to 2005 then you can't restore to an older version (the database version has been upgraded)
the only thing i can see (if you absolutely must downgrade) is to build a new server and use a data transfer tool to move the data to the new server... but again, why?