Login and Click Suppliers in the top left corner.
From the Suppliers menu on the Blue Bar Select 'Manage Purchase Assignings.'
Find the Catalog for the Supplier you wish to change (Search or page through the list)
Click Work With when you find the supplier Catalog.
The following page is split in two and displays supplier product details on the left and nominal details on the right.
In the supplier product details grid, tick the product(s) to be assigned / changed.
Then tick the nominal code to assign the products to.
Click the Match button in the centre of the screen to assign.