insert yesterday date data and today date date into table

In my application I want insert only  yesterday data and todays data .rightnow in that code im deleteing all data from table and inserting again into the table with new data.it takes long time when I run the application .old data will never change so I want to delete onlye yesterday data and insert yesterday and today data.but I don't know how to insert only yesterday and today data .Please any suggestions?

  string connectString = ConfigurationSettings.AppSettings["AagentDB"];
                            //// Establishing the databse connection  
                            SqlConnection conn = new SqlConnection(connectString);

                            string sqlDel = "DELETE FROM QBPurchaseorders dbcc checkident(QBPurchaseorders,reseed,0)";
                          //string sqlDel = "delete from  QBPurchaseOrders  WHERE OrderDate >=CONVERT(nvarchar(10),getdate()-1,101) AND OrderDate < CONVERT(nvarchar(10),getdate(),101)";
                            SqlCommand cmdDel = new SqlCommand(sqlDel, conn);
                            conn.Open();
                            cmdDel.ExecuteNonQuery();

                            IPurchaseOrderRetList purchaseList = (IPurchaseOrderRetList)response.Detail;
                                                       IPurchaseOrderRet curPurchase;

                            SqlCommand cmd = new SqlCommand("QB_insertPurchaseOrders", conn);
                            DateTime wantDate;
                            DateTime? adjustedWD, orderDate;
                            //, bookedDate