def replace_nan(df, value):
    """Replace ``NaN`` for a value.
    
    Args:
        df (pd.DataFrame): Dataframe.
        value (int or str): Value
    
    Returns:
        pd.DataFrame: Dataframe with the replaced value.
    
    Examples:
        >>> df = pd.DataFrame({'letters':['a','a','c'], 'numbers':[1,2,None]})
        >>> df_return = replace_nan(df, 7)
        >>> df_return
          letters  numbers
        0       a      1.0
        1       a      2.0
        2       c      7.0
    """
    return df.fillna(value)