Update

//  Create a Map to store all the data of a Purchase Order
List<Object> purchaseOrderList = new List<Object>();
Map<String, Object> purchaseOrderMap = new Map<String, Object>();
    purchaseOrderMap.put('DateString', '2021-11-11T00:00:00');
    purchaseOrderMap.put('DeliveryDate', '2021-12-06T00:00:00');
    purchaseOrderMap.put('Status', 'AUTHORISED');
    purchaseOrderMap.put('PurchaseOrderID', '2fef007e-1979-4634-9bb7-87c4559a0038');

    //  Initializing the Contact
    Map<String, Object> contactMap = new Map<String, Object>();
    contactMap.put('ContactID', 'c84ebb15-0a8d-4145-933f-203e68d014a0');
    purchaseOrderMap.put('Contact', contactMap);
    //  Initializing the Purchase Order 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('UnitAmount', 100);
        lineItemMap.put('Quantity', 10);
		lineItemMap.put('AccountCode', '310');
        lineItemMap.put('LineItemID', 'fd9bd48c-9f10-44de-aee4-c6185b2868c3');
        lineItemList.add(lineItemMap);
        
    //  Adding the Line Items to Purchase Order
    purchaseOrderMap.put('LineItems', lineItemList);
    purchaseOrderList.add(purchaseOrderMap);

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

    Map<String, Object> finalReqMap = new Map<String, Object>();  
    finalReqMap.put('version', '1.0'); 
    finalReqMap.put('action', 'updatePurchaseOrder'); 
    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);