To change the version of your server, you need to manually upload a server jar
You can easily upload a server jar by following the steps below
Delete the file called minecraft_server.jar (if on our Pterodactyl panel, the file is called server.jar)
Download the server jar version you want from the internet (a good place is https://serverjars.com)
Upload the server jar to your server
Rename the server jar minecraft_server.jar (if on our Pterodactyl panel, rename the file server.jar)