To install a datapack you have to put the datapack folder that you downloaded or created into the /world/datapacks/ folder on your server, where "world" could also be renamed if you changed the name in the properties.
You can access this folder by going into your control panel and using the file browser in the "FTP" menu tab on the top. When you're there just upload the datapack and run the /reload command if your server is currently running