Change product costing method is essentials based on your type of business. Different industries follow different methods for ascertaining cost of their products. The method options available in Odoo are

1. Standard price(Manually inputed value in the cost field)

2. Average price(The value becomes read only and system automatically computes and applies)

3. Real price( Either the last incoming shipment based cost or the first incoming shipment based cost will be used for valuation.)  

In odoo you have the flexibility to use any of the costing method to do your inventory valuation.
Refer the below screenshot that will help to do the settings. Follow the navigation Purchase>> Products>> Products to apply changes.