Open
What it is
The Open button under Project file in the left sidebar. Its tip says Open a project file.
What to enter
Choose a project file that was made with Export.
What it changes
It replaces the project on screen with the one in the file and marks it as having unsaved changes. Nothing reaches the house or the app until you save and deploy.
If it goes wrong
If the file is not a project file, you see Invalid file and the screen is unchanged. Opening a file throws away unsaved changes on screen, so save first.