Python replace specific value with some constant

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Replace NaN in dataframe with ” 

df_confirmed = df_confirmed.replace(np.nan, '', regex=True)
Fill NaN with another column 
Day  col1  col2
1    1     a
2    NaN   b


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

Day  col1  col2
1    1     a
2    b     b
Replace NaN with 0 

df['DataFrame Column'] = df['DataFrame Column'].fillna(0)
Replace all NaN in df 

df.fillna(0)