Laptop heatsinks are generaly not something you can change/upgrade with laptops, never heard of it myself apart from replacments.
First thing you need to do is open your laptop and clean it of dust. Number one problem with Fan/Heatsink combo's is DUST! So clean that out 1st and also try re-applying some thermal past, Arctic Silver 5 is good. Then see what temps you are getting, if they are still high, I would suggest a possibly problem with the laptop and would look at getting it returned, any warranty left on it?
My advise for the future is, STAY AWAY FROM DELL PRODUCTS!