my_text = 'Aello world'
my_text = my_text.replace(my_text[0], 'H')
print (my_text)
>>> mystring = 'Hello World'
>>> mystring = '*'*len(mystring)
>>> print(mystring)
***********
>>> a = '&#'
>>> print a.replace('&', r'&')
&#
>>> print a.replace('#', r'#')
&#
>>>