In order for a return to be possible, the status of the purchase order must be completed. If the status of the purchase order is 'received', there won't be a return to supplier button.
Once you generate a supplier invoice, the button will appear.
Once you click on it and confirm, a window will appear where you can change the quantity of products you are returning. You can return the whole or partial quantity and if you are not returning some products that are on the list, just change the quantity to zero.
When you post the document, a new document will appear', and it will keep the same number as the purchase order, but it will have 'returned' as a status.
Now, when you open the stock card for this product, you will be able to see that the returned quantity has been deducted from your stock.