You can install Spigot via our version downloader or by uploading the jar yourself.
To install via the version downloader, select Spigot from the drop-down and select the version you need.
Then just restart the server.
Download the version you need from here: https://getspigot.net/ (opens in a new tab)
Upload the jar via the file manager.
Once uploaded, set the jar in the server overview page.