The screen needs to be replaced - the labor to take the screen apart will far exceed the cost to replace the panel.

Hi avsii,

Welcome to the community.

Please check the post below which would help you with this issue:

http://dell.to/13MYl1r

If you have any further queries, please feel free to contact us.

Thanks and Regards
Sandeep P
#iworkfordell

Dear Sandeep,

I have exactly the same problem and I must say these ARE NOT dead pixels. This is dirt/dust stuck behind the gorilla glass. 

Now I would greatly appreciate if you could provide us with a manual or instructions on how to remove gorilla glass.  

The touch panel is glued to the screen - you must use a heat gun to soften it and then pry the screen loose.  There's a video on how to do that, here:

Great, except for the video is about Dell xps 15z but NOT about Dell xps 15 (l521x), which doesn't have a metal frame covering the glass. So the video is of no help

The disassembly process is the same.  The glass is glued to the LCD panel and you will need to follow the same process.

There is a high probability of cracking the glass when trying to remove the panel - just be prepared for that, and the glass isn't sold as a separate part.