iOS device UDID

As of macOS Catalina (macOS 10.15), iTunes isn't available. But you can get the UDID using Finder. Connect your iOS device to your computer and open Finder. You should see your iOS device listed under Locations on the left side of the Finder window. Select your iOS device. Details about your device will appear on the right side of the Finder window. At the top will be an icon of your device showing your device's name (such as "Bob's iPhone 11"). Under that will be type of device, disk size and available space, and your battery level. Tapping on the line with the batter level a few times will cycle through other details about the iOS device. One of these will include the UDID. Once displayed you can right-click on the UDID and select Copy to copy the serial number and UDID to the pasteboard.

For those with Xcode, you can get the device's UDID from the Devices and Simulators window. Select the Devices tab on that window. Select the iOS device on the left side under Connected. In the detail pane on the right you will see Identifier: followed by the UDID. Right-click on the UDID to copy it to the pasteboard or share it in some desired fashion.

Old answer for those still using iTunes

Connect your iPhone to your computer and open iTunes. Select your device in iTunes so you can see the serial number. Click on the serial number. The UDID will take its place. You can copy & paste the UDID that's displayed on the screen.

If one of these tutorials don't do it for you, then nothing will!

http://whatsmyudid.com // For old iTunes versions

or use this - http://get.udid.io // For new iTunes versions

or use this - Working https://udidiphone.com for UDID, IMEI, Serial Number, OS Version, Build Number

If nothing is showing up in iTunes, you have to open up Xcode, go to the devices list (Window -> Devices and Simulators) and then click on the devices tab. Under devices, select your device in the left sidebar and it will show your udid in the main window. You can also just click use for development and it will register your device for you.