Which file? This software comprises of a lot of files that need to be placed on your server in a coordinated way. This process is facilitated by the comand-line instructions mentioned above:
Flarum utilizes Composer to manage its dependencies and extensions. So, before installing Flarum, you will need to install Composer on your machine. Then run this command in the location where Flarum should be installed:
composer create-project flarum/flarum . --stability=beta
You can perform this process on your local computer if you just want to have a look into the software, but there is no streamlined process of uploading the contents to your server via FTP.