why isnt the name of a column cahnging using pd rename code example

Example 1: rename column name pandas dataframe

df.rename(columns={"old_col1": "new_col1", "old_col2": "new_col2"})

Example 2: how to change column name in pandas

print(df.rename(columns={'A': 'a', 'C': 'c'}))
#         a   B   c
# ONE    11  12  13
# TWO    21  22  23
# THREE  31  32  33