To simply save the file under its existing name, click the SAVE button in the toolbar. The prior save, under that name, will be overwritten. In essence THAT is "Save Project File."
If the project has previously been saved, with a title, that name will appear in the Save Project File AS box. If it was never named, it with say "UNTITLED" in the file name box.
Change it there. It's good practice to periodically use SAVE AS, incrementing the file name. That way, earlier saves are retained.