I am currently using Photoshop to change a white product to black. Even though I have selected the parameters and used feather tool for selection I am facing issue with having jagged edges as seen in the image. Is there something I am doing it wrong?
I would use a vector mask to make such an edit. The beauty of a vector mask is that the paths remain editable, and are almost perfectly smooth with no jagged pixels. So if you mess up, it's easy to edit the path.
It might be worth your while searching for a tutorial on vector masks in Photoshop if you are unfamiliar with how to use them. You might also want to find a tutorial on using the Pen Tool. There are quite a few skills you need to perform this successfully and there isn't enough room to cover it all here.
Here is an example of a vector mask in action.
This is not really an issue with "jagged edges" so much as it's an issue of a poorly constructed Layer Mask.
For most layer masks on photos, you will not want hard edges. Many selection methods, by default, result in hard edge selections.
After you create a mask, sometimes you can merely double-click the mask and adjust some mask options to soften the edges:
This may work in some cases.
In my experience, there's no such thing as a quality layer mask for a photo without having to manually take a brush to refine the mask. You zoom in, and paint where you need to correct the mask.
In this instance, you need to smooth the curves and expand the mask slightly while at the same time lowering the overall opacity around the edges of the mask.
Here, in green, you can see where I've painted additional areas to smooth the mask:
After manually refining the mask, the overall appearance will be much closer to realistic.
Unfortunately I don't have the original image to show here, the best I can do is use your edited image containing the poorly masked dark color. Because of the underlying poorly masked dark color, it is somewhat difficult to show a polished final image. So the below is far from perfect due to this, but it's closer. (I wasn't prepared to "retouch" the photo to eliminate the existing mask errors. So, I just worked around the ones I could.)
No matter what any web site or online tutorial claims, quality masks take practice and manual interaction (painting) in almost all instances. It's actually more rare that all you need do is create a selection add a mask and be done.
If you isolate the black object, eg have it on its own layer then make a touch bigger and use the blur option, from memory its one of those that you can adjust the settings while viewing the preview of the changes. Tweak so it softens the edges and covers the area you need.