Before manage Stock make sure your Inventory Option is enable (How to enable stock)
Step 1 : Click on Products Menu & select Stock Option
Step 2 : On Stock page you will see a list of all products, Current Stock & New Stock Fields for all Products
– To increase Stock for the product Enter the positive Quantity Value you want to increase in the New stock Field for that product. Like current Stock is 25 & you want make is 35 so you need to Enter 10 To New Stock Field & click on save button.
– To decrease Stock for product just Enter negative Quantity Value you want to decrease in New stock Field for that product. Like current Stock is 25 & you want make is 20 so you need to Enter -5 To New Stock Field & click on save Button.
Step 3 : After Clicking on save Button you Can see Stock Values are Updated for product which you have added New Values