Update

//  Create a Map to store all the data of a Invoice
List<Object> invoiceList = new List<Object>();
Map<String, Object> invoiceMap = new Map<String, Object>();
    invoiceMap.put('Id', '13');
	invoiceMap.put('SyncToken','2');
	invoiceMap.put('CustomerRef',new Map<String, Object>{'value' => '9'});

    //  Initializing the Invoice Line Items
    List<Object> lineItemList = new List<Object>();
        Map<String, Object> lineItemMap = new Map<String, Object>();
		lineItemMap.put('LineNum',1);
        lineItemMap.put('Amount',100);
        lineItemMap.put('Description','Description Updated');
		lineItemMap.put('DetailType', 'SalesItemLineDetail');
		Map<String, Object> salesLineItemDetailMap = new Map<String, Object>();
            salesLineItemDetailMap.put('Qty',2);
            salesLineItemDetailMap.put('UnitPrice',50);
			salesLineItemDetailMap.put('TaxCodeRef' , new Map<String, Object>{'value' => 'TAX'});
        
        lineItemMap.put('SalesItemLineDetail', salesLineItemDetailMap);
        lineItemList.add(lineItemMap);

    //  Adding the Line Items to Invoice
    invoiceMap.put('Line',lineItemList);
    invoiceList.add(invoiceMap);

//  Initializing the RequestJSON Data to be passed.
    Map<String, Object> reqJSONMap = new Map<String, Object>(); 
    reqJSONMap.put('invoices' , invoiceList); 
    String reqJSON = JSON.serialize(reqJSONMap); 

    Map<String, Object> finalReqMap = new Map<String, Object>(); 
    Map<String, Object> reqOptions = new Map<String, Object>(); 
    finalReqMap.put('options' , reqOptions); 
    finalReqMap.put('version' , '1.0'); 
    finalReqMap.put('action' , 'updateinvoice'); 
    finalReqMap.put('RequestJSON' , reqJSON); 
    finalReqMap.put('skipDML' , false);
    //Required, if QuickBooks Multi-Org is enabled in Breadwinner. 
    finalReqMap.put('qbOrgId', '!aa00TM'); 
    System.debug('Request ::: ' + finalReqMap);

//  Placing a request to Breadwinner Global API
    Map<String, Object> respMap = breadwinner_qbo.BreadwinnerQBAPI.call(finalReqMap);
    System.debug('Response ::: ' + respMap);