or use the menu item or the keyboard shortcut - Ctrl+S [1]Now that you’ve loaded your file into Sigil, you should save it.
You should always save right after loading a new file – even if it was an EPUB to start with – to make sure there are no problems with saving.
Since the file you are editing is now an EPUB (even if you loaded an HTML file to start with), when you save in Sigil you will be saving an EPUB formatted file (although it might be missing some details that you will need to add later).
To save your EPUB, click the Save icon or use the menu item or the keyboard shortcut - Ctrl+S [1]
If you loaded an HTML file, your EPUB won’t have a name yet – so you will be prompted to enter a name for your file. It will be given a .epub extension since it will be saved as an EPUB file.
You can click the Save icon 
to save your EPUB at any time – and you should save often.
Notice that the title of your book – at the top of your window – is updated (e.g. to TestBook.epub) and that the status bar briefly shows "File saved":
You now have an EPUB file – but don’t stop yet!
To make sure your ebook conforms to EPUB standards, you need to add a bit more information as shown in the following tutorials.
1 - If you wish to retain the original version of your ePub unmodified there are two further save options - File→Save As which creates a copy with a different name, and File→Save A Copy which creates a copy of the EPUB with _copy appended to the name.