There are situations where instead of setting the whole batch as damaged, you only require to set a single item from this batch as damaged. Here is how it can be done:

1. Go to My Inventory
2. Locate the item
   • Open the Product Detail Screen
3. Go to Batches tab and select the Batch No which you wish to modify 
   • Double click on the Quantity field
   • Insert the new stock Quantity (which would be the previous batch quantity minus the quantity of damaged items)

Since there is no other way to set a single item as damaged without setting the whole batch as such, after changing the quantity of the initial batch the only step left is creating a new batch (same batch) with the status 'damaged' that contains only as many items as have been damaged: 

1. Click Add New 
2. Enter the details of the new damaged batch (the same details as your modified batch)  
   • Batch No -  for example, 123456789Damaged
   • Sell by / Expires On - the same dates as the original batch. For example, 02 May 2017 / 10 Aug 2017
   • Quantity - the quantity that has been damaged. For example, 110 - 109 = 1
   • Status - DAMAGED
3. Click Add 
4. Click Save 

For further information on inventory batching, please refer to this documentation.