To find the serial number on Beosound Explore, please look at the bottom of the speaker. Here you can find the serial number (SN followed by an 8-digit number).
Alternatively, if your Beosound Explore is added to the Bang & Olufsen app, you can find the 8-digit serial number under "Product information -> Product settings -> About"
It can also be found on the bottom of the Beosound Explore packaging or on your invoice if purchased from https://www.bang-olufsen.com