The function replaces the last character of the input string with the next character in the alphabet. If the last character is 'z', it is replaced with '}'

[CODE]:
```python
def approx_function(s):
    if s[-1] != 'z':
        return s[:-1] + chr(ord(s[-1]) + 1)
    else:
        return s[:-1] + '}'
```