Replacing the power supply is probably the easiest task inside the computer case. Simply open the case, unplug all power cables - notice the broad connector and the one with two pairs or black and yellow have a securing clip! You might want to take a picture of it before unplugging, maybe even print it and make some notes how the cables are routed and where they're plugged in. Or then not... Anyway, you can't plug the cables wrong without violent force!
Since it's a Dell, I'm not 100% sure whether the measurements are standard. At least the cables are much shorter than default for better airflow. If I were you, I'd take the old PSU with me to the local builder shop. They may have a second hand spare for next to nothing. If not, they can make sure you'll get one with matching plugs - there's been some redesigning during the last decade or so, modern power supplies having connectors for SATA disks and 4/6/8 pin video cards.
And of course the easiest way is to take it to a professional. Again, I'd prefer the local builder shop selling second hand items. They might let you look how it's done and the charge should be nominal unless they do some extra maintenance.
