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('InvoiceID', 'c473fb66-9e4c-445a-9855-bba92d6629e9');
    invoiceMap.put('Status', 'AUTHORISED');
    invoiceMap.put('Reference', 'Test Invoice');

    //  Updating the Invoice 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','c638e3bd-3427-485f-a5f4-ea8b2b4742d4');
        lineItemMap.put('UnitAmount',100);
        lineItemMap.put('Quantity',10);
		lineItemMap.put('AccountCode', '200');
        lineItemList.add(lineItemMap);
        
    //  Adding the Line Items to Invoice
    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>(); 
    Map<String,Object> reqOptions = new Map<String,Object>(); 
    finalReqMap.put('queryParameters' , reqOptions); 
    finalReqMap.put('version' , '1.0'); 
    finalReqMap.put('action' , 'updateInvoice'); 
    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);