For big uploads (worlds, modpacks) use SFTP with an app like FileZilla or WinSCP. Your exact details are in the SFTP card on your server's Settings tab, with a button to open them straight in your SFTP app.
- Host: the address shown in that card (
45.45.216.173) - Port:
2022 - Username: your panel username, a dot, then the server's ID, e.g.
steve.a1b2c3d4 - Password: your panel password
Smaller files can go straight through the Files tab in the panel: drag and drop to upload, and edit config files in the browser.