You have different options of finding the serial number:
• On a label on the product packaging.
• In the Bang & Olufsen app under "Product settings" - "About". Please note that Beoplay A2 needs to be connected to the app and switched on.
• On a label behind the back plastic grills: Gently remove the grill by popping it off all the way around the edges of the speaker and look for the sticker with serial number.