import pandas as pd

# Creating a DataFrame
df = pd.DataFrame({'COLUMN_1': [1, 2, 3], 'COLUMN_2': [4, 5, 6]})

# Renaming columns using a lambda function
df = df.rename(columns=lambda x: x.lower())  # Lowercase column names

# Print data frame
print(df)

>>>

      column_1  column_2
0         1         4
1         2         5
2         3         6