This is a trivial exercise to get you started with Git and GitHub. All you need to do is edit exercise.txt to replace Hello World with Hello Mars! Don't forget the exclamation mark.
Once you've made your changes and saved them, you just need to commit and push, and then head back to your repository page on GitHub to check if you've passed the test.
A detailed description of how to do this exercise is provided on the FoNS Git documentation page.