Replace string in many files in a folder

[metro17]

This code changes a string in 1 text file in python

s = open("sample.txt").read()
s = s.replace('abcd', 'efgh')
f = open("mount.txt", 'w')
f.write(s)
f.close()

How do I change a single string ex: from abcd to efgh in 100's of text files under a single directory.

Thanks

I also tried this.It runs without errors but changes nothing in the text files.
I ran the code under the directory under which the files are stored.

import glob,os,fileinput
from glob import glob
for filename in glob('*.LIC'):
    with fileinput.FileInput(filename, inplace=True, backup='.bak') as file:
        for line in file:
            print(line.replace('abcd', 'efgh'),  end='')