There is a tech support forum here for the 57XX series notebooks.
In that forum there is a link to the service manual for the 5720...http://www.notebookforums.com/thread190971.html
Go there and download the service manual. It will show you how to disassemble the 5720 down to removing the heat sink. From there you can simply exchange the CPU.
BTW....if you need any parts for your 5720 I have one I'm parting out. I also have an internal second hard drive enclosure for the 5720 if you're interested. It's an ATA enclosure that swaps out with the DVD drive allowing you to have a second internal hard drive instead of an optical drive. I have to price it but I think they're pretty cheap. I can also include a working 120MB drive already mounted.
I also have 2x1GB RAM if you need any.