File permissions as root

my coder also install flarum as a root, so how to fix it. www-data group and user can fix the problem?
what is the default flarum's permissions?

All files can remain root, except for:

./storage (recursively)
./assets (recursively)

Those two are the bare minimum for Flarum to work I think. In order to make Bazaar work you also need:

./
./composer.json
./composer.lock
./vendor (recursively)

You can change ownership like so,

For non-recursive:

chown www-data: <path>

For recursive:

chown -R www-data: <path>

If you still run into issues consider allowing more actions on the directory, I recommend using:

chmod -R 0755 assets storage