def replace_string_chars(s1, pos, s2):
    """
    Replace characters in a string at the specified string position (index).
    @param s1: Input string for replacement.
    @param pos: Input string index of replace.
    @param s2: - Replacement string.
    @return: New resulting string.
    """
    len2 = len(s2)
    if(pos <= len(s1)):
        if(len2 > 0):
            s1 = s1[0:pos] + s2 + s1[pos+len2:]
    return(s1)