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The standard price can not be updated directly.
One way to update the standard price is to fill the fields Future Price (MBEW-ZKPRS) 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.