VB.net: how do I change the column mapping for an excel sheet when the columns differ?

Private Sub submitBtn_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles submitBtn.ServerClick
            
 
            
            Dim fileName As String = zipFile.PostedFile.FileName
            Dim ConnectionString As String = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source='" & fileName & "';" & "Extended 
 
Properties=""Excel 8.0;HDR=YES;IMEX=1"""
            Dim excelConnection As System.Data.OleDb.OleDbConnection = New 
 
System.Data.OleDb.OleDbConnection(ConnectionString)
            excelConnection.Open()
 
            Dim excelCommand As New System.Data.OleDb.OleDbCommand("SELECT column1, column2, column3 From [Sheet1$]", 
 
excelConnection)
            Dim dr As DbDataReader = excelCommand.ExecuteReader
            Dim mapping1 As New SqlBulkCopyColumnMapping("column1", "column1")
            Dim mapping2 As New SqlBulkCopyColumnMapping("column2", "column2")
            Dim mapping3 As New SqlBulkCopyColumnMapping("column3", "column3")
 
            Try
 
                Using destinationConnection As SqlConnection = New SqlConnection(DBAccess.GetConnectionString2)
                    destinationConnection.Open()
 
                    Using bulkCopy As SqlBulkCopy =      New SqlBulkCopy(destinationConnection)
                        bulkCopy.DestinationTableName =  "dbo._temp_table"
 
                        bulkCopy.ColumnMappings.Add(mapping1)
                        bulkCopy.ColumnMappings.Add(mapping2)
                        bulkCopy.ColumnMappings.Add(mapping3)
 
 
                        Try
                            ' Write from the source to the destination.
                            bulkCopy.WriteToServer(dr)
 
                        Catch ex As Exception
                            'result.Text = ex.Message.ToString
                            Response.Write("Exception:" & ex.ToString & "<br>")
                            'Response.End()
                        Finally
 
                            dr.Close()
                            destinationConnection.Close()
                        End Try
 
 
                    End Using
 
                End Using
 
                result.Text = "File upload successful. Thank you."
            Catch ex As Exception
                Response.Write("Exception:" & ex.ToString & "<br>")
                Response.End()
                
                Exit Sub
            Finally
                excelConnection.Close()
 
            End Try
        End Sub