Changing Java Version
To find your instance please follow this guide: https://wiki.creeper.host/books/minecraft-java-edition/page/finding-your-instances
First, you will need to make sure that the instance you are changing the version on has been stopped (you can find out how to stop your server https://wiki.creeper.host/books/minecraft-java-edition/page/startingrestartingstopping-your-server)
If you need to change the Java version that is used to run your server you can follow the step by step guide below.
Click on Settings as highligted in the image below
On the settings page click on System