From the keyboard
Use the key combination Control+S.
If the spreadsheet has not been saved previously, then each of these actions will open the Save As dialog. Here you can specify the spreadsheet name and the location in which to save it.
|| If the spreadsheet has been previously saved, then saving will overwrite the existing copy without opening the Save As dialog. If you want to save the spreadsheet in a different location or with a different name, then select File > Save As.