First, create a Stock Adjustment as you normally would and then select the batch to apply the stock adjustment to. You will be able to choose from all the batches containing this variant via the dropdown.
Here's how to create a stock adjustment for batch tracked products:
- Under Stock Control > Stock Adjustment, click the Create New drop-down menu and select 'New Stock Adjustment'.
- Fill in the location and reason for the stock adjustment. Add product(s) and quantity change, and click 'Create'.
- Add the existing batch IDs or create a new batch in the popup and click 'Create'