If you only have one or a few products to correct, you can update the inventory directly from the Product form. Go to the menu Warehouse > Products > Products. Go to the Inventory tab, Stock and Expected Variations section, and click Update next to the Quantity On Hand field. 

Enter the New Quantity on Hand, the location and, if required, a serial number. Make sure to always enter the total quantity available; Odoo will consider the current quantity as the Theoretical Quantity and the newly entered quantity as the real Quantity. Only the difference between Theoretical and Real Quantity will be posted in the stock.

Click Apply to confirm the inventory correction; Odoo will create a physical inventory for you with the name of the product as a reference.

The correct levels of your product are now in your stock locations. A simple way of verifying this is to reopen the product form to see the quantities available in stock.

Another way is to check the inventory concerned from the menu Warehouse > Inventory Control > Inventory Adjustments.

When you need to make many changes, you can correct a wrong inventory through the menu Warehouse > Inventory Control > Inventory Adjustments. Refer to the topic https://www.odoo.com/forum/Help-1/question/How-can-I-record-my-annual-inventory-55267 for more information.