Unique Device Identifier (UDID) is a 40-character string of code for each iOS powered devices from Apple. UDID is made up of a mixture of letters and numbers. You can easily find out a device’s UDID in iTunes.
Finding UDID using iTunes:
Plug your iOS powered device into your computer (Mac or PC)
Go to device summary page in iTunes.
Click on the text that says Serial Number. It should change to UDID.
Pressing Command key ⌘ + C (in mac) or Ctrl + C (in windows) at the same to copy the UDID code to clipboard.