@mohitvyas what you are describing sounds like a damaged AMOLED display. You will need to replace the display.
@martin52280 Since iFixit does not yet have a specific guide for that, use this video. Replacement displays are readily available at many online stores. Make sure that you purchase the one that has the frame already attached. The No Frame LCD replacement is a real pain to replace.
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!
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