Update: for Odoo 18, you need to select a Product, then choose Adjust Valuation from the Actions Menu.

Update: for Odoo 17, you need to group by Product, then expand the product to see the button.

Update: for Odoo 16, the Valuation Report is available in Developer Mode and you need to group by Product, then expand the product to see the button.

Available in Version 13, 14 and 15 for products with FIFO or AVERAGE costing and quantities on hand:

Prior to revaluation, each Widget is $10 and there are 25 in stock:

Revaluation:

After revaluation, each Widget is $8: