The commingled setting can be updated in Seller Central.
Steps to Update Commingled Setting
Log in to Amazon Seller Central.
Navigate to Settings (gear icon in the top right) and select Fulfillment by Amazon.
Scroll down to the FBA Product Barcode Preference section.
Click Edit.
Select your preferred barcode type:
Amazon barcode: Disables commingling (requires you to label items with FNSKU stickers).
Manufacturer barcode: Enables commingling (uses the existing UPC/EAN).
Click Update.