"I want to calculate a Landed Cost of one product" - think of the split methods as "templates". The allocations are calculated based on one of the standard methods - equal, by quantity, by current cost, by weight, by volume - but you can modify the valuation adjustments before validating the Landed Cost if you would like to modify the amounts each product will increase in value by because you have a custom landed cost split method. More at https://www.odoo.com/forum/help-1/landed-cost-splitting-method-199354

"how can we change the delivery order cost" - you can't change the delivery cost of something you have already sold. If you process a Landed Cost for products you have already sold, we will move the amount to cost of goods, since the delivery order cost has already been moved to cost of goods. Remember, delivery cost only stays in the interim account until you create a Customer Invoice, then it moves to cost of goods.