The serial number is printed on a label placed inside the connection panel on the bottom of the Beoplay M3.
If your Beoplay M3 is added to the Bang & Olufsen app, you can also find the 8-digit serial number under "Product information -> Product settings -> About." The Bang & Olufsen app is available for download from the App Store and Google Play Store.
Alternatively, the serial number can be found on the packaging of Beoplay M3.
