Open the Settings app
Tap General
Tap About
Scroll to IMEI (or IMEI1/IMEI2)
View or copy the IMEI shown on-screen
Open the Phone app
Dial *#06#
View the IMEI displayed on-screen
Connect iPhone to a computer
Open Finder (macOS) or iTunes (Windows)
Select the iPhone
View the IMEI in the device information section (if shown)