new_columns = {"col1": "1st", "col2": "2nd"}
df_new = df.rename(columns=new_columns)

"""
 col1 | col2 | col3            1st | 2nd | col3
   a  |   d  |     g              a  |  d  |  g
   b  |   e  |   h        =>      b  |  e  |  h
   c  |   f  |   i              c  |  f  |  i
"""