To change the number of partitions in a table, first unpartition the table using alter table...unpartition.
Then use alter table...partition, specifying the new number of partitions. This does not move the existing data in the table.
You cannot use the partition clause with a table that is already partitioned.
For example, if a table named historytab contains 10 partitions, and you want the table to have 20 partitions, enter these commands:
alter table historytab unpartition
alter table historytab partition 20