Change filename of Excel-files based on the content of a cell
The good practice is to finish your code with

if __name__ == '__main__':
     sys.exit(main())


which has the advantage of returning an error code to the system (for example for batch processing of related programs).

It means 3 things for changing your code:


import sys
moving your command rename_excel_files(r"C:\Users\Stewie\Documents\excel_folder") to the new created main() function.
adding an return statement to the main() function with value 0 in the case of the success (and optionally something else for the failure):  


So the result will be something as

import os
import sys
import xlrd
    ...
    ...
    ...

def main():
    rename_excel_files(r"C:\Users\Stewie\Documents\excel_folder")
    return 0

if __name__ == '__main__':
    sys.exit(main())


(See "3.3.1 Use sys.exit in your script to return proper error codes" in the book Writing Idiomatic Python by Jeff Knupp, 2013 - or directly from our BDFL - in All Things Pythonic - Python main() functions by Guido van van Rossum
May 15, 2003)