Lowercase first N charactersΒΆ

S[:N].lower()

Lowercase first n characters in a string.
S = 'W3RESOURCE.COM'
print(S[:4].lower() + S[4:])    # w3reSOURCE.COM

See also

https://www.w3resource.com/python-exercises/string/python-data-type-string-exercise-47.php