Read the Excel file and store it in a datatable variable. Use Build datatable activity to create a new datatable. Map the columns in the new datatable in build datatable activity. Use for each row in datatable activity to iterate the extracted datatable. Use the add data row activity to add the data in to the new datatable. For slno column take a counter variable and give that counter variable in the add data row activity. After add data row activity increment the counter value.
Finally, use write range workbook activity to write the new datatable data in to the excel file.