#sftp

Use FileZilla or WinSCP with protocol SFTP, port 2022, the username from SFTP Details and your Game Panel password.

The Files tab has Create Directory, Upload (files, folders or from a URL) and New file, plus a code editor and zip/unzip, so you can manage server files in your browser.

Zip your world folder, stop the server, upload and extract it in Files (or use SFTP), then set level-name in Properties to the folder name and start.

Plugins tab → Browse Plugins → pick a platform (Modrinth, CurseForge, Hangar or SpigotMC) → search → Install → restart. Or upload any plugin .jar to the plugins folder in Files.