Goto 'Catalog' then scroll over to 'Products'
Goto the desired product and click 'Associations'
Next to 'Manufacturer' in the drop down list select '- Choose (Optional) -'
Note: Be sure to keep track of what manufacturer supplies this product as this option removes the association with the product and the manufacturer on the website. It is recommended to keep a separate note. Alternatively, can have the said manufacturer listed as a supplier of the product as not to remove the association between the manufacturer and the supplier. A note can be placed in the supplier description box with "MANUFACTURER" typed in bold for any admin of the site to understand. This is a 🙂 alternative option as a product can have more than one supplier. Be sure to do this before removing the association with the '- Choose (Optional)-' option and clicking save.
Click 'Save'.