UDIDs are identifying numbers that are unique to individual iPhones and iPads (and ipodtouch devices, for that matter). They’re 40 characters long, and made up of a mixture of letters and numbers.
How to find out your iPhone’s UDID
1. Plug your iPhone into your Mac, and go to iTunes (or start it up if it isn’t already open).
2.Select the iPhone’s icon at the top-right corner of the window, underneath the search box. (If you’ve got more than one ios device plugged in, this button will say ‘2 devices’ instead – press it and then select the iPhone.)
3. You’ll now see the iPhone’s summary page. (If it’s showing apps, podcasts or whatever, select Summary from the top bar of tabs.) You can see the iPhone’s capacity, phone number and serial number in the top box; click on the serial number and this will change into the UDID.
4. After the serial number changes into the UDID, pressing Apple + C at the same time will copy the UDID to the pasteboard. Alternatively, right-click it and select Copy.
5. Find a blank document and paste the UDID into it by pressing Apple + V.