change a attribute in dataframe

df = pd.DataFrame({'A': [1, 2, 3],
                   'B': [400, 500, 600]})
new_df = pd.DataFrame({'B': [4, 5, 6],
                       'C': [7, 8, 9]})
df.update(new_df)
df
   A  B
0  1  4
1  2  5
2  3  6