Gui if this is a VPS, your Apache config will have a property named DocumentRoot where you customize where the website is. On nginx it's called root.
If this is a managed hosting, there is often a place where you can edit the path to the website files in the management panel.
If the managed hosting doesn't let you customize the path or if you are installing in a subfolder, you can follow https://flarum.org/docs/install.html#customizing-paths to remove the public folder of Flarum so that the DocumentRoot doesn't have to be customized.
