Some black spot on my mobile screen

@niteshverma what you are describing actually sounds like a damaged screen. We would need to know what this looks like. Post some pictures of what your screen shows with your Question. Again, what you are describing sounds like a bad screen. If that is the case, you will need to replace it. Adding images to an existing question

Since iFixit does not yet have a specific guide for that use something like this video Replacement screens are readily available at many online stores

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has a device with the same issues as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!