Most cases have 2 screws in back holding the side panel on. Remove the panel (usually the left one gives access).
Look at the picture on
this page. That is what your power supply (PSU) looks like.
Unplug all the cords leading from your old power supply to the motherboard and disk drives - remember where the cords go because your new power supply will look almost exactly like the old one.
Most cases have 4 screws in the back holding the power supply in place. Remove the old power supply, and screw the new one into place.
Reconnect the cords, put the side panel back in place, and your computer is fixed.
When shopping for a new PSU, be sure to get one that is the same size and watts as your current PSU. According to the Dell website, the Dimension 2350 ships with a 250w PSU. Replacing it with a more powerful one will likely cause overheating issues.