Access Apple Business Account:   https://business.apple.com/

Login with userid and password.   Opening screen looks like this.  Select Devices.

A list of Devices registered in Apple Business is displayed.  You can enter the serial # of the device you want to remove from Apple Business

Click on the Device on the left side and the Device3 details will display on the right.  To remove device, click on 'Release from Organization' at the top.