how to replace values by another in python code example

Example 1: replace values of pandas column

df.loc[df['column'] == 'column_value', 'column'] = 'new_column_value'

Example 2: replace value of dataframe with another column

df['col1'] = np.where(df['col1'] == 0, df['col2'], df['col1'])
df['col1'] = np.where(df['col1'] == 0, df['col3'], df['col1'])