df['column name'] = df['column name'].replace(['old value'],'new value')
df["column1"].replace({"a": "x", "b": "y"}, inplace=True)
# change value in column_where_to_change and in the row where column_name == column_value
df.loc[df['<column_name>']=='column_value', '<column_where_to_change>'] = '<new_value>'