Update

//  Create a Map to store all the data of a Bill
List<Object> invoiceList = new List<Object>();
Map<String, Object> invoiceMap = new Map<String, Object>();
    invoiceMap.put('InvoiceID', '192d2166-7cfa-4f30-89c1-9ad1dc1f55df');
    invoiceMap.put('Status', 'AUTHORISED');
    invoiceMap.put('InvoiceNumber', 'Test Bill');

    //  Updating the Bill Line Items
    List<Object> lineItemList = new List<Object>();
        Map<String, Object> lineItemMap = new Map<String, Object>(); 
        lineItemMap.put('Description', 'Line Item Description Updated');
        lineItemMap.put('LineItemID', '1294d14b-30e6-4c76-85f5-d66de2c02b83');
        lineItemMap.put('UnitAmount', 100);
        lineItemMap.put('Quantity', 10);
		lineItemMap.put('AccountCode', '200');
        lineItemList.add(lineItemMap);
        
    //  Adding the Line Items to Bill
    invoiceMap.put('LineItems', 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>(); 
    finalReqMap.put('version', '1.0'); 
    finalReqMap.put('action', 'updateBill'); 
    finalReqMap.put('RequestJSON', reqJSON); 
    finalReqMap.put('skipDML', false); 
    //Required, if Xero Multi-Org is enabled in Breadwinner. 
    finalReqMap.put('xeroOrgId', '!aa00TM'); 
    System.debug('Request ::: ' + finalReqMap);

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