It's true that, with a removeable back and battery, there's perhaps less need to dismantle the Lumia 520 in the first place, but there may well be a particular component that has failed and the video below shows how to get to it - even down to replacing the screen or digitiser.
In the video, he covers:
01:42 Back Cover Off
02:54 Mainboard Removal
03:45 Mic & Usb Port
04:21 Display Removal
05:29 Headphone Socket/Jack
06:13 Earpiece Speaker
07:09 Proximity Sensor
07:46 Digitizer & Frame Separation
10:53 Genuine vs Aftermarket Digitizer
11:34 Double sided tape or Glue
Another terrific demonstration from LE55ONS, giving us all confidence in doing something similar, even if we can't get near his speed and style!
I've dismantled numerous Symbian-based and Android devices after watching LE55ONS at work, but so far I haven't tackled a Windows Phone. What about you? Have you had need to get into this sort of thing?
I'm guessing that most viable devices are still in warranty, mind you, in which case best just Subscribe to his channel, ready for when you need the videos at some time in the future!