You can always be working in your app to create content and improve it. However, changes are not auto published. You will have to publish to make your changes visible to your users.
Thankfully, publishing your app is very simple.
In ProgramBook at the very bottom of the left sidebar navigation is Publish Book.
Here you will see a list of your sessions and any potential errors. As long as everything looks good go ahead and click Publish at the bottom.