It’s not currently possible to return part of a product in Breww.
For now, the best option is to make a manual stock adjustment on the single-bottle/can product (from the Actions → Stock adjustment menu). This will increase the stock level of the product. You could also make a manual adjustment on your duty return to reduce this by the duty amount on these returned products so that you’ve not overpaid duty on them when they are sold. To do this, go to Beer duty → View date. Then scroll down to the Adjustment button.