To create a stock adjustment click on Stock

Under the Stock click on Add Stock Adjustment

Enter the Name for stock adjustment

Enter the Reason for stock adjustment

Click on Add Product to add product Information

Adding a +ve number to add that quantity to the product stock

Adding a -ve number will remove that quantity from that product stock 

Click Save to save the Stock Adjustment

Once saved system will display updated stock quantities

Before stock adjustment:

After stock adjustment: