You can rebuild global index partitions in two ways:
Rebuild each partition by issuing the ALTER INDEX REBUILD PARTITION statement (you can run the rebuilds concurrently).
Drop the entire global index and re-create it. This method is more efficient because the table is scanned only one time.
For most maintenance operations on partitioned tables with indexes, you can optionally avoid the need to rebuild the index by specifying UPDATE INDEXES on your DDL statement.