How do I only replace color for one area?

Just adding to what Dave said.

Always try to get as far as possible with global adjustments first. If the subject appears too dark, dirty or muddy, try to lighten the whole image up with a Curves adjustment, before doing anything else. Surprisingly often, that gets you what you want, or at least very close.

Only then, and only if absolutely necessary, use selections, masks or other targeted tools to go after specific parts.

A very common beginner's mistake is to start with a poorly exposed image, and then go right at selecting and masking. This will always, no exceptions, end up looking unnatural and artificial, or plain bad.

I'm with Trevor. This will never look good - because the black background is reflected in the object itself. You cannot "remove" that without extensive manual labor. I guarantee you, reshooting will be orders of magnitude quicker!

Also, a sharp outline is an absolute no-no here. That looks really ugly and unattractive.

Just recently, a book designer attempted something like this with a couple of my images. I went so far as to demand that they hold the presses for reasons of copyright violation. It's just unacceptable.

One way would be choosing Select > Color Range. After selecting the black background and clicking OK, the back colors on your actual product will be selected. But since you have a selection you can use the selection tools, like the Lasso tool, to remove the black pixels you don't want in the selection.

If you post an example of one of your photos with a black background, I will give more precise directions.