white light in the upper part of screen, what is it?

It looks like something is pressing on the back of the screen. Something may have got between the LCD and the LCD back plate. Hard to say exactly what's going on there.

The only way to fix it, is to have someone open it up to inspect it. You could try Apple, but they are unlikely to do that kind of diagnosis.

Or have an independent repairer have a look at it to see if something is in there.

When did this occur or has it always been there? Is it on every screen? Did this occur after the phone was dropped?

As a 3rd party repairer SunOfRa78 failed to tell you that Apple reserves the right, and does this most of the time, to refuse to deal with an iPhone that has been opened by a non-authorized repair person. If you go to a 3rd party repair person and they are a hack and damage your phone further your only recourse is to buy a new phone, you will not be eligible for an out-of-warranty replacement for $299 USD.

I would take it to Apple first (make a Genius Bar appointment before you go) and have them test the phone. Then you can decide what you want to do based on their advice. As to finding a professional 3rd party repair facility you are going to have to do some research as there is no organized group offering certifications.

Are you able to post a photo of it?