To change the price on a DNCB 501R vending machine, locate the control panel and access the programming mode by pressing the designated buttons (usually a combination of "Service" and "Clear"). Once in programming mode, navigate to the pricing settings for the selection you want to change. Input the new price using the keypad, then exit the programming mode to save the changes. Always consult the specific manual for your vending machine model for detailed instructions, as steps may vary slightly.