God, I really hate to have to write this post. Anyone below the age of 40 has known this since they were toddlers. But there are those among my friends who are unclear on the concept, so here goes: The difference between the "File...Save" and "File....Save As" options in any File menu is that if you choose the first option while you are editing a file you will be overwriting (updating) the file. If you choose the second option you will be given the opportunity (via a pop-up menu) to create a new version of the file, with a different name (and in a different storage location if you prefer).
Why is this important to know? One, because it's always nice to know how things work, and so you won't be confused about why there are two options.
And two, because if you're at all unclear about the difference, you should remember to use "Save As" all the time. Here's why. When you choose "Save As" you get the pop-up menu I mentioned. And it will automatically show you the current name of the file and where it is located. So you can examine the info and then decide if you want to make any changes. You don't have to, you can just leave it alone, but you have the option. Not so when you use "Save". When you use "Save" it just writes over the file. No going back.
When you use "Save As" and you choose a new file name, one that is different from the original, you will be creating a new file and leaving the original untouched.
I often use "Save As" a lot when I'm working on a project where I am making incremental changes to a file and I want to be able to go back to an earlier version if I mess up. I just keep creating new copies named things like "cat_photo_1", "cat_photo_2", "cat_photo_3", etc.