The standard price in the Material Master can’t be updated in a direct manner. A great way to update it is to fill the fields Future Price MBEWZKPRS and the Effective Date MBEW-ZKDAT for the material on the accounting view. Next, go to Logistics → Materials Management → Valuation → Valuation Price Determination → Future Price → Activate.
TCODE MR2B, program RMMR2100. Lastly, run the BDC that was created to update the standard price.