Goto Magento Backend > Inventory Management > New Stock Adjustment
Choose your Location and Fill the Reason
Press Start Adjust Stock
Click Add Products to Adjust Stock and choose some products inserting to the below list
Input the Change Qty according to different with the Old Qty and the real quantity of the location
Press Adjust. You will see a new list with Old Qty, Change Qty, and Adjust Qty according to the real quantity.
Click Save & Apply
You can click Save and come back to continue working later
You can Print List of Stock Adjustment by click Export to have a CSV file