Remove the left ear cushion by turning it counter-clockwise. Look at the edge and find the serial number (SN followed by an 8-digit number). To mount the ear cushion again, make sure to align the mark inside the headphone with the mark on the ear cushion and then turn it clockwise.
Alternatively, if your Beoplay H9 3rd Gen are added to the Bang & Olufsen app, you can also find the 8-digit serial number under "Product information -> Product settings -> About".
