The function replaces the second character of the input string with the character that comes immediately after it in the alphabet, if the second character is a letter. If the second character is the last letter of the alphabet (z or Z), it wraps around to the first letter (a or A). If the second character is not a letter, the function returns the same input string.

[CODE]: 
```python
def approx_function(s):
    if len(s) > 1 and s[1].isalpha():
        next_letter = chr(((ord(s[1].lower()) - ord('a') + 1) % 26) + ord(s[1].lower()) + (ord('a') - ord('a')))
        if s[1].isupper():
            next_letter = next_letter.upper()
        return s[:1] + next_letter + s[2:]
    return s
```
Let's test the approximation function on a new input.

COMMAND: PYTHON(./temp/function.py "apples")