The function takes a string input and replaces the first character with another character by incrementing its ASCII code by a fixed value. If the first character is a digit or special character, it remains unchanged.

[CODE]:
```
def f(x):
    if x[0].isalpha():
        new_char = chr(ord(x[0]) + fixed_value)  # Replace "fixed_value" with the appropriate value
        return new_char + x[1:]
    else:
        return x
```