SQL & PL/SQL

Drop index un1 ;
create index un1 on xx_test (a1,a2,a3) ;

I dont want to drop the index is it possible to alter?

yes you can alter index with "alter index" syntax to change.
You can alter index for :-
Rebuild or coalesce an existing index
Deallocate unused space or allocate a new extent
Specify parallel execution (or not) and alter the degree of parallelism
Alter storage parameters or physical attributes
Specify LOGGING or NOLOGGING
Enable or disable key compression
Mark the index unusable
Start or stop the monitoring of index usage

-----You cannot alter an index's column structure.---------

For more details check in the documentation
http://download.oracle.com/docs/cd/B10501_01/server.920/a96521/indexes.htm#518

[ALTER INDEX|http://download.oracle.com/docs/cd/B19306_01/server.102/b14200/statements_1008.htm#SQLRF00805] is beautifully documented.

Thanks,
Karthick.