To find the serial number on Beoplay H100, please remove the left ear cushion. Look at the top half edge and find the 8-digit serial number. Please note that it may require lighting at a very specific angle to view the serial number.
To mount the ear cushion again, simply place it back on the magnets and make sure that it is correctly aligned.
Alternatively, if your Beoplay H100 are added to the Bang & Olufsen app, you can find the 8-digit serial number under "Settings -> About -> Serial number", on your invoice, and on the Beoplay H100 packaging.
