Alkir when I update the first time I disabled all third part extension, removed all Flarum files (after a full folder and database backup) than installed Flarum as new and just replaced conf file and asset folder. For making Flarum point to the new public folder you can configure the domain from your panel to point in the new public folder or you can try to use the .htaccess suggested.
I was getting 500 error when I add the line to override all. For me that official solution did not work so if you customized Apache patch or .htaccess try to revert it back.
