Connect the iPhone to a Mac or PC
Open Finder on macOS Catalina or later
Open iTunes on Windows or macOS Mojave or earlier
Select the connected iPhone
Click the device name or serial number to reveal the UDID
Right-click the UDID and copy it
Use Xcode on Mac and open Window > Devices and Simulators
Select the iPhone and copy the Identifier field
Use a third-party UDID retrieval website on the iPhone if needed
Install a configuration profile or app that displays the UDID
Use Apple Configurator on Mac to view the device UDID