I may be misunderstanding what I'm reading in the instructions, but will removing items from the downloaded product file remove them from my database?

I have this installed, but have been afraid to try it because it took forever to get my database fixed last time I tried to upload products.

Basically I need to know if I'm understanding the functions correctly.

I downloaded a "complete" file.

I made changes in excel, and saved as the same type of txt file as I downloaded.

If I upload the changed file, does it overwrite existing products with the changed items of the same model number? or does it upload everything in the file as new items? I don't want duplicates.

By the same token, if I upload a file that only contains new items, and does not have existing store items in it, will it delete items that are not in the new file?

I just don't want to mess stuff up... lol

Thanks!