I ended up having to make a separate project, as when I put "Hi Annabelle!" and "Bye now." into the original project, it still displayed "Hello World!" in the resulting Command Prompt.
Annabelle, if you think about this logically that cannot be the right approach. You don't/must not have to create a new project in order to alter something like the output from a piece of code you have already written.
Why don't you go back now to the original project and have a stab at making the edits again? If that still produces the wrong output as you said, paste the code and we'll have a look at it.