Get a JDK 11 here : https://jdk.java.net/11/ and setup your IDE for working with it.
Run the test in src/test/java/DojoTest with JUnit, they should always works during the Dojo
Refactor the code in src/main/java/Dojo.java with the new functionalities of Java 9, 10, 11
You can follow the slides here : https://slides.com/loicmathieu/java-9-10-11 the refactor comment contains the number of the slide.
Read the code and each time there is a //TODO refactor ... refactor it ;)