Where do I find the serial number on my Beoplay M3?

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." 

Alternatively, the serial number can be found on the packaging of Beoplay M3.