Can someone please explain to me how I can change the field 'Product Brand' to 'Manufacturer'?
Log in to your admin panel and navigate to Stores -> Attributes -> Products and create a new attribute called "Manufacturer" or edit the already existing "Product Brand" attribute.
If you want to remove the attribute "Product Brand" from the options. Navigate to Stores -> Attributes -> Attribute Set and remove the "Product Brand" attribute from the corresponding attribute set.
Note: If you create a new attribute, don't forget to add it to attribute set!