public with sharing class CusOrderData {
    public List<Cus_Order__c> order{get;set;}//show Order Data 
   public Cus_Order__c orderdata{get;set;}
     public Object items{get;set;}
 
    public CusOrderData(){
        orderdata=new Cus_Order__c();
        order=[SELECT Name,Activated__c,Order_Start_Date__c,Order_End_Date__c,Invoice_Date__c,
               Project_Start_Date__c, Project_End_Date__c, Down_payment_amount__c FROM Cus_Order__c];// Show Order Data   
         items = [SELECT Id, name FROM Cus_Order__c where Id=:ApexPages.currentpage().getparameters().get('Id')];
        system.debug(items); 
    }   
   public PageReference Save(){
           upsert order;
               PageReference pnext = new PageReference('/apex/CusOrderData?id='+orderdata.id);
                pnext.setRedirect(true);
            return pnext;
    }
    public void Edit(){                    
    }
}

VF page:
<apex:page controller="CusOrderData" lightningStylesheets="true" tabStyle="Account">
    <apex:form >
        <apex:pageBlock title="Order Edit Page">
               
                <apex:pageBlockSection >                
                <apex:inputField value="{!orderdata.Name}" />
                <apex:inputField value="{!orderdata.Activated__c}" />    
                <apex:inputField value="{!orderdata.Invoice_Date__c}"/>
                <apex:inputField value="{!orderdata.Project_Start_Date__c}"/>
                <apex:inputField value="{!orderdata.Project_End_Date__c}"/>
                <apex:inputField value="{!orderdata.Order_Start_Date__c}"/>
                <apex:inputField value="{!orderdata.Order_End_Date__c}"/>
                <apex:inputField value="{!orderdata.Down_payment_amount__c}"/> 
                    </apex:pageBlockSection>
            
            <apex:pageBlockButtons >
                <apex:commandButton value="Save" action="{!Save}"/>
            </apex:pageBlockButtons>
        </apex:pageBlock>
    </apex:form>
</apex:page>