Python Pandas Multi-index Column: Set first level of header column as index

I have a pandas dataframe with multi-index column headers that looks like this:

I would like it to look like this:

>Solution :

Try stack and swaplevel:

>>> df
            A        B      
            1  2  3  1  2  3
11/12/2021  1  2  3  4  5  6

>>> df.stack(level=0).swaplevel()
              1  2  3
A 11/12/2021  1  2  3
B 11/12/2021  4  5  6