When you want to save your progress, you have two choices. If you intend to continue, you can press the Save button in the top left of the document window, visible while the toolbar is shown.
Press Save. Any changes you have made will be saved and you will be returned back to the workbook ready to continue editing. Or if you are finished, you can press the Close button at the top left of the workbook window. Sheet2 HD will prompt you to save or discard your current changes.
Press the Save button to save the changes and close the workbook. The new workbook file will be shown in the files list. Sheet2 HD lets you easily revert back to a previously saved version of the file. For more information see Reverting to a previous version of a file.