Yellow color merges on the top screen

Has it been recently replaced?

If so, sounds like the metal Sheild on the flex maybe too high/low pushing pressure on the lcd.

Is the yellowing large, or small?

A Yellow dot is typical on poor quality backlights on replaced screens. If you have not had a replacement screen and the screen you have is the original one another possibility is that if the phone has received a blow (typically from a drop ) it may well have created a small dent that has placed the screen under pressure thus in turn placing the backlight under slight pressure and that is showing through with a yellow patch.

An easy way is just to connect a replacement screen but dont seat it back in place. Boot the iPhone and see if the yellow patch is gone if it has then power down and disconnect the battery and screen again. Then examine the housing any even tiny dents etc can cause the issue. If you dont have the tools to remove those dents its quite cheap to get a new housing