
Your UDID is in the hardware, it's a hash composed of various serial numbers and other values. You can't physically change your UDID.

And in order to generate fake UDID,you can use UDID faker,which requires jailbreaking. UDIDFaker is a jailbreak app for the iPhone/iPod Touch that changes your default UDID of an app to anything you want. You can hit the random button for a random UDID. It also has a button to change your UDID back to default of the app.