1. Go to [Inventory] [Settings] [External Locations] and click on the Supplier that you want to send the goods back to.
2. In the top left of your screen, click the green [+ New] button, then on [Stock Adjustments].
In the [Destination] dropdown, select the Stock Location that you want to transfer the goods from. Then click [Save].
3. Enter the Quantities (in cartons or whatever is stated in the 'Count by' column) in negative, as you are returning the products to the Supplier.
E.g. Put '-1 in' the [Quantity] column to return 1 carton of 24 back to a Supplier, or '-5' to return 5 cartons of 24. Then click [Complete].
Note - Return items to Suppliers in CARTONS, not by bottles, and make sure the quantity you enter is in NEGATIVE as it is returning to a Supplier and not transferring to the stock location.