You should use the "Save As" option when you create your document for the first time, and the "Save" option to save the changes made to an existing document.
When you create a new document, using the "Save As" option allows you to specify the file name, file type, and location where the document will be saved. This creates a new file that can be easily identified and accessed later.
When you make changes to an existing document, you should use the "Save" option to save those changes to the same file. This updates the file with the latest changes and ensures that you don't lose any work.
Using both options appropriately ensures that you can keep track of your files and have the latest versions of your work saved.