By default your minecraft will be setup with the latest version of Paper. Paper is a minecraft server jar that has enhancements for performance.
The Latest Information on Paper can be found here https://papermc.io/downloads/paper
To see your server settings for Minecraft Version, Build Number and Java Version, Login to the Nitro Panel and Click on the Startup Tab.
To Change the Version of Minecraft
4. Then Click On the Settings Tab and Select Reinstall Server. This may delete your saves so be sure to backup any data you may want to keep.
5. Once that has completed, make sure your server is online and connect to the game.