Human: replace substitute text in string. Assistant: 
Here is a Python program to replace substitute text in string strings

import string

def _replace_substitute_text_in_string(string, old, new):
    """
    Replace substitute text in string.
    """
    return string.replace(old, new)

# Enter the string
string = "This is a string"

# Enter the old and new text
old = "This"
new = "This is a new string"

# Calling the function
print((_replace_substitute_text_in_string(string, old, new)))

#Example Output
# This is a new string is a string

#Remember to inspect code for bugs.