@abdullahiqbal "black spots on display screen" is most certainly a bad display. Replacing it would be the only fix.
Since iFixit does not yet have a guide for that, use use videos [link|https://drive.google.com/file/d/10Dpv6b6H4VIbPUwaTKXumVsYmBMHIu8F/view?usp=sharing|like this one] as your guide.
Replacement screen are readily available. Just do an online search and find a vendor to your liking. Prices are not to high neither.
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 the same problem 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!