I have got two data frames.

```
>>> import pandas as pd
>>> import numpy as np
>>> df = pd.DataFrame({'A': [1, 2, 3], 'B': [400, np.nan, 600]})
>>> print(df)
   A      B
0  1  400.0
1  2    NaN
2  3  600.0

```

and

```
>>> new_df = pd.DataFrame({'B': [4, 5, 6], 'C': [7, 8, 9]})
>>> print(new_df)
   B  C
0  4  7
1  5  8
2  6  9

```

How can I update `df` by `new_df` to fill NaN values? I would like to get following:

```
>>> print(df)
   A      B
0  1  400.0
1  2    5.0
2  3  600.0

```