Hi Dada,
According to the official Dell Service Manual, your computer's display can only be replaced as an entire unit; the LCD and/or LVDS cable is not separately replaceable. It sounds to me like the issue you're seeing is most likely one of those two parts, but either one requires the replacement of the entire display.
Here's a link to the Service Manual that will show you how to replace the display assembly.
[document|25544|XPS 15 Service Manual - iFixit]
It's hard to say exactly what replacement screen you need, as there seem to be a few different ones available, but generally it looks like they're running anywhere from about $75 USD for a used one on eBay, on up to about $400 or more for a new replacement part.