You can switch from one undo tablespace to another using Oracle Enterprise Manager Database Express (EM Express).
To switch the undo tablespace:
In EM Express, go to the Undo Management Details page, as described in "Viewing Undo Information".
Click Switch Undo Tablespace.
The Switch Undo Tablespace page appears.
In the Switch to Undo Tablespace field, select the name of the undo tablespace you want to switch to. This field includes the names of available undo tablespaces for the database.
For example, if the current undo tablespace is named UD1 and you want to switch to the undo tablespace named UD2, select UD2 in the Switch to Undo Tablespace field.