I am trying to alter the column name-

What should be the cause of error??????

It is stated in the Oracle response above: CHANGE is not valid option of ALTER TABLE statement.
Maybe, before using any SQL statement, you should familiarize with its proper syntax instead of inventing your own.
ALTER TABLE statement is described in SQL Language Reference book, which is available with other Oracle documentation books e.g. online on http://tahiti.oracle.com/
Just pick the one for your Oracle version.

Basically I want to change the column name to lowercase, I tried remaining the column but its not useful as it would take the duplicate columns.
How can I change the column name to lowercase?

Changing name = renaming. Not sure what you mean with "duplicate".
Rename it. Oracle identifiers are case insensitive, interpreted as uppercase. When you want to use mixed case or include any special character except underscore (_), dollar sign ($) or pound sign (#), you have to put them between double quotes.
Note that you will have to always use double quotes when referencing that column. Why do you want to do this at all????
(yes, those four question marks are intentional, because it is really not a good idea. definitely not)

You may read more facts about Oracle identifiers (they use the term "Object Names and Qualifiers", just search for it) in the same book I suggested you in the previous post.

I couldn't agree more.