Ok, its in this order:
Open your case and find that green air duct and remove it. (should have release tabs on it)
Then, look for the (usually green) color lever that when flipped loosens the tension on the heatsink so you can lift it off.
Look underneath the heatsink to see if there is either thermal paste (usually white) or a sticky pad (usually dark gray) there.
If there is a sticky pad, you dont have to do anything. If there is white thermal paste, wipe it off and apply a new VERY THIN film in the same
area that the old stuff was.
Then theres the CPU. There should be a small silver metal (or tan plastic) lever that when flipped, will release the CPU.
Lift the CPU straight up so as not to bend any pins (gently now).
take your new CPU and look at the pins. You will notice that there are some pins missing from one of the corners. Match that up to the CPU socket and gently ease it in.
After the CPU is resting in its socket, press down gently on the center of the CPU and throw that little silver (or tan plastic) lever back to the lock position.
Place the Dell factory heatsing ( not the retail heatsink fan combo) on to the CPU and throw the green lever to lock it.
Replace the green duct.
close your case and boot up.
You should be good to go.
"WHEW"