How to change degree of partitioned index

Hi

alter index test_idx6 rebuild partition test1 tablespace BIG_INDEX parallel 4;

11:50:03 SQL>  SELECT INDEX_NAME, DEGREE FROM DBA_INDEXES WHERE INDEX_NAME='TEST_IDX6';

INDEX_NAME                     DEGREE
------------------------------ ----------------------------------------
TEST_IDX6                1

In dba_indexes, the degree of test_idx6 is still 1.

How can I change the partitioned index test_idx6 back to parallel 1?

Thanks,

Tracy