You've got a fabulous not-new, refurbished laptop or desktop. Here's how to find the serial number so that you can let your insurer know or make a note in your asset register.
For an Apple Mac: From the Apple menu (the little apple logo) in the corner of your screen, choose About This Mac. You will find your serial number here.
For a Dell: Search for Support Assist in the search bar, and open. Click on the Service Tag near the top right corner. You will find your serial number here.
For a HP: Search for HP System Information in the search bar, and open. You will find the serial number at the bottom of the screen.
For a Lenovo: Search for Lenovo Vantage in the search bar, and open. Click on the Device Details tab. You will find your serial number here.