Configuring the Project Structure
You can configure the structure of your project with ⌘; (macOS), or Ctrl+Alt+Shift+S (Windows/Linux).
The most important thing to note is that if you're using a build system such as Maven, or Gradle, they will override settings in your Project Structure.
You can change your Java SDK here, and your Language Level. The Java SDK is the collection of tools to develop the application, and your language level defines the coding assistance you get in the editor, and by default, the language level is also your target bytecode version.
You can also manage your existing Java SDKs here and download new ones from various vendors.