You may need to find the Unique Device Identifier as you may be a developer or your developer asked for it, so you need to have iTunes installed on your computer to find the UDID. to find your iPhones Serial Number,IMEI number you can find it directly from the phone. The UDID is attached with the motherboard of the Apple devices so it is permanent and should not be changed.
How to get the Serial and IMEI number
You can also find the Integrated Circuit Card Identifier (ICCID) also on the same screen.
Go to Settings => General => About and scroll down to view them
How to Get UDID and ECID
To find UDID, ECID (Exclusive Chip ID or Electronic Chip ID) you need to connect your Phone to computer and open iTunes. Go to the summery tab of the iTunes where it will display the details of your phone.
If it is not visible click on the phone icon
On the summery tab it will be displaying Serial Number
Just Click on the serial number, it will display the UDID there
If you click again it will display ECID and on next click Model Identifier
If you want to copy the UDID , just right click and copy
Used an old iPhone 4 to get the screens.