Loop through all the characters in your phrase and increment the character value by '0' - 'a' (yes, the difference between two characters).
In Python, this would look something like this:
phrase = input('?')
shift = ord('0') - ord('a')
chars = list(phrase)
length = len(chars)
i = 0
while i < length:
chars[i] = chr(ord(chars[i]) + shift)
i += 1
phrase = ''.join(chars)
print(phrase)