Replacing the power-supply is relatively simple. The power-pins are designed in a way that they fit only when being inserted properly. That makes screw-ups less likely. The one thing you need to be careful with is touching the components of the computer. Before touching anything, wash your hands with soap for 20 seconds (removes grease) then before getting into the case touch the metal chassis (removes static electricity that may have built up). Even the slightest discharge of static into any electrical component can render it useless (Figured that one out the easy way).
I have read a few complaints that the dell power supply doesn't fit the dell case. However, those complaints were few. My guess is that it will fit just fine. If not, you can always return it to the retailer opting to try a different PSU or to receive a full refund.
By the way, the Power-Supply is rather self-explanatory. It provides power to all your system's components.
