Can we assure that no one can change the device UDID by hacking the OS of an iPhone or iPad. Because we are using that UDID for authentication of our users. We want to know that hackers wont able to change it gain access to our system.
Don't do this! And, no u can't tell that.
UDID is deprecated in userland.
Jailbroken UDID fakers do exactly that. They intercept calls to UDID via mobile substrate and return their own value. The only thing you could do is detect jailbroken phones and block them completly.
Also how would you tell if they had a proxy between the app and the internet connection and changed this value on the fly.