Add a method to a Apex Class, Execute the method to update the Accounts queried (Know how to write and execute method in a class. Also, you will get the answer for error like 'System.DmlException: Update failed.....theField__c field is mandatory:[theField__c]')

Apex Class:

public class GoldAccountsUtility {

public static void updateGoldAccounts() {

        // Get the 2 oldest accounts

        Account[] oldAccounts = [SELECT Id, Description,Region__c FROM Account ORDER BY CreatedDate ASC LIMIT 2];

        // loop through them and update the Description field

        for (Account acct : oldAccounts) {

          acct.Region__c = 'APAC';

          acct.Description = 'Gold Account';

        }

        // update Accounts

        update oldAccounts;

    }

}

Execute from Anonymous Window:

GoldAccountsUtility.updateGoldAccounts();

Details : https://www.youtube.com/watch?v=7WxxBEPGWU0