Here is how you can remove stock:

 
Step 1: Click on 'Menu' on upper left corner

Step 2: Click on 'Inventory Management'

Step 3: Choose the item to which you want to add Stock 

Step 4: Click below Sublist or Variant price and details

Step 5: Click on 'stock available'

Step 6: Enter a number (less than stock available number) and click on 'Remove' 

Step 7: Click on Update

Step 8: Click on Save on upper right corner