Here is How To find your iPhone’s UDID (Unique Device Identifier).
You may be asked to provide UDID to Apple when registering a development device.
Step 1. Connect your iPhone to a computer via USB and launch iTunes.
Step 2. Choose your iPhone from the navigation menu at the top left.
Step 3. Select Summary section.
Step 4. Click the serial number of your device to display your UDID.
Step 5. To copy UDID to your clipboard press Command+C or Control+C on Windows OS