To save your project work, use the Project ▸ Save menu, the Save button on the main toolbar or press Ctrl+S.
When you save a project for the first time, a dialog window appears, asking you where to save the project file and how to name it.
If you want to save your project as a different file, use Project ▸ Save as or press Ctrl+Shift+S. This can be used to create different versions of your project.
In addition to undoing individual changes, you can also decide to revert all the changes you have made since the last time you saved your project. To do so, use Project ▸ Revert.