Visit this page in Safari on your iOS device to install a profile that sends us your UDID automatically. This is the easiest and most reliable way.
Note: This is currently the only way to find an Apple Watch's UDID.
Connect your iOS device to your computer and open iTunes.
Select your iPhone, iPad, or iPod touch when it appears in iTunes.
Click on the serial number to switch to your device's UDID.
Copy the UDID using ⌘C, right-click, or the Edit menu.
Return to the registration page and paste your UDID.