In the following question, out of the four alternatives, choose the word which best expresses the meaning of the given word and click the button corresponding to it.
Use SAVE when you want to update the lastly preserved or to revise an existing document or file with the latest content.
Whereas
Use SAVE AS when you are creating a new document/file or an existing document/file to a new location. This will keep the original document untouched in its original format and create a new document with a new name.