import urllib.request

nowa_wiadomosc = input()

url = 'link do pobrania pliku'
file_name = 'nazwa_pobranego_pliku.py'
urllib.request.urlretrieve(url, file_name)

import nazwa_pobranego_pliku
with open(file_name, 'r') as file:
    data = file.readlines()

data[0] = 'message = "{}"'.format(nowa_wiadomosc)

with open(file_name, 'w') as file:
    file.writelines(data)