Is there a way to easily rename the name of the 2nd column of Pandas DataFrame in the case below?

```
a = pd.DataFrame({'a':[1, 2], 'b':[3, 2]}).groupby('a', as_index=False).agg({'b':['sum', 'count']})

```

I tried the answer from [this question](https://stackoverflow.com/questions/43759921/pandas-rename-column-by-position) but this dataframe has a multi-level column name (i.e. `a.columns[1]` is `["b","sum"]` instead of a single string like `"bsum"`). The following code won't work.

```
a.rename(columns={a.columns[1]:'new_name'})

```

For my case, it is not very easy to figure out the exact column name so I want to have a way to change the name of the column based on it's position.

Thanks in advance.