Shrinking Undo Tablespace Size

Undo space once allocated will be available for reuse but will not be deallocated to the OS. The best way to shrink Undo tablespace is to switch to a new Undo tablespace and drop the old Undo tablespace. The steps are:

1. Create a new undo tablespace of the same size (larger or smaller) depending on your database requirements.
SQL> CREATE UNDO TABLESPACE "APPS_UNDOTS1"
DATAFILE
        '+HGPRODDATA' SIZE 16384M ,
        '+HGPRODDATA' SIZE 16384M ;
		
SQL> CREATE UNDO TABLESPACE "APPS_UNDOTS2"
DATAFILE
        '+HGPRODDATA' SIZE 16384M ,
        '+HGPRODDATA' SIZE 16384M ;

2. Switch to the new Undo tablespace:
SQL> ALTER SYSTEM SET UNDO_TABLESPACE = APPS_UNDOTS1 SCOPE=MEMORY sid='HGPROD1';
SQL> ALTER SYSTEM SET UNDO_TABLESPACE = APPS_UNDOTS2 SCOPE=MEMORY sid='HGPROD2';

3. Check the status of the undo segments and determine if all the segments in the old undo tablespace are offline.
If there are Undo segments with a status other than OFFLINE in the tablespace to be dropped, we need to wait till they become OFFLINE. You may have to wait for the duration of the tuned_undoretention (from v$undostat) to ensure all Undo segments have become OFFLINE.

SQL> select tablespace_name, status, count(*) from dba_rollback_segs group by tablespace_name, status;
SQL> select status,segment_name from dba_rollback_segs where status not in ('OFFLINE') and tablespace_name='UNDOTBS2';
SQL> select tablespace_name, status, count(*) from dba_rollback_segs where tablespace_name='UNDOTBS2' group by tablespace_name, status;

4. If all the Undo segments in the old Undo tablespace to the dropped is of status OFFLINE, then drop the tablespace.
SQL> select tablespace_name, status, count(*) from dba_rollback_segs group by tablespace_name, status;

5. Verify and then drop:
SQL> drop tablespace UNDOTBS2 including contents and datafiles;

Add Space to the Undo Tablespace
For increasing / resize undo tablespace there are two options :
Resize the existing undo datafile
Add new undo datafile to the tablespace.

引用：https://www.thegeekdiary.com/how-to-resize-the-undo-tablespace-in-oracle-database/