as per docs, Use the DBMS_SPACE_ADMIN package  
something like 

execute sys.DBMS_SPACE_ADMIN.TABLESPACE_MIGRATE_TO_LOCAL('SYSTEM');

This operation requires that:

1.	Migrate all other dictionary tablespaces to locally managed before
	migrating the system tablespace.
	The database has a default temporary tablespace that is not SYSTEM
2.	There are no rollback segments in dictionary-managed tablespaces
3.	There is at least one online rollback segment in a locally managed 
	tablespace, or if using automatic undo management with (9i), 
	an undo tablespace is online.
4.	All tablespaces other than the tablespace containing the undo space (that
	is, the tablespace containing the rollback segment or the undo tablespace)
	are in read-only mode
5.	There is a complete cold backup of the database ( to be in the safer side)
6.	The system is in restricted mode