To increase or decrease the quantity of a product allocated to a particular batch you will need to create a Stock Adjustment.
Go to the Stock Control section and click on Stock Adjustment. Fill in the details as you normally would. Click Create.
You will now need to select the batch to apply the stock adjustment to. You will be able to choose from all the batches containing this product, via the dropdown.
Select the batch and click create.
You will now be able to see the batch that this stock adjustment was applied to.