Download a Datapack
Download your desired Datapack from websites like Planet Minecraft
Log In to the Control Panel ↗

Select Your Server

Stop the Server

Click on the Files tab

Open the World Folder

Open the Datapacks Folder

Upload the Datapack File
Upload the downloaded datapack file. Make sure it is a .zip file.

Start the Server

You now know how to install a datapack on your server.