Click the small pencil (Change Supplier) next to the ingredient name. A dropdown appears with No supplier and every supplier; when you pick one that already has receipt data for this ingredient, a Price preview (unit cost from the imported COGS) is shown. Click Save: the ingredient's supplier is updated (and, when a COGS price exists, its price too), you see Ingredient Vendor updated with success, and the report reloads so the ingredient moves under its new supplier. The pencil is not offered for items flagged as sub-recipes. Only admins can do this.