Getting the UDID of your device using iTunes 12 on both Mac and Windows can be done using the steps below:
1) Launch iTunes
2) Connect your iOS device
3) Click on “Serial Number” and the UDID will be revealed
4) Copy the UDID using the menu choices Edit->Copy
The UDID is used by iOS developers in order to create builds which you can install on your iOS device. The two videos below show the process step-by-step on Mac and Windows.