>>> import pandas as pd
>>> 
>>> df = pd.DataFrame(['1/2/2020','12/31/2020','20-Jun-20'],columns=['Date'])
>>> df
         Date
0    1/2/2020
1  12/31/2020
2   20-Jun-20
>>> 
>>> df['Date'] = pd.to_datetime(df['Date'])
>>> df
        Date
0 2020-01-02
1 2020-12-31
2 2020-06-20
>>> 
>>> df['Date'] = pd.to_datetime(df['Date']).dt.strftime('%y-%m-%d')
>>> 
>>> df
       Date
0  20-01-02
1  20-12-31
2  20-06-20
>>>