Dell laptop chargers are designed to deter tampering. When assembled, the casing for the charger is moulded around the internals of the charger so that opening it will cause the case to break open instead of pop open. It may be better to buy a new charger if the old charger is faulty or see if there may be a warranty available on the old charger before tampering with it. Tampering with an item covered under a warranty will void the warranty.
Unplug the Dell charger from power outlet and the laptop.
Place a prying tool such as a large flathead screwdriver into the slot located along the side trim of the charger. Newer charger models will have a slot at the side to fit a flathead screwdriver for prying. Older models will not ,and you may need to cut it to open.
Twist the flathead screwdriver so that the casing cracks open. You may need to pry open the case. Note that the casing will break open rather than "pop" open.
If you have an older laptop charger that does not have a spot to insert a tool to pry it open, you may need to cut along the trim line around the whole charger to open it. Use a knife or box blade to do this.
Once you've opened the charger, most repairs would have to be done by a professional due to most components being soldered together instead of screwed or bolted. The soldering has to be done in a way so that components do not burn or melt.