pandas change value in dataframe at a certain place code example

Example 1: change dataframe value by index

x    y
A  NaN  NaN
B  NaN  NaN
C  NaN  NaN

df.at['C', 'x'] = 10

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'])