df_confirmed = df_confirmed.replace(np.nan, '', regex=True)

Day  col1  col2
1    1     a
2    NaN   b

df['col1'].fillna(df['col2'])

Day  col1  col2
1    1     a
2    b     b

df['DataFrame Column'] = df['DataFrame Column'].fillna(0)

df.fillna(0)