Best practice to create a new undo tablespace.
his method includes segment check.
 Create pfile from spfile to edit
SQL> Create pfile='/tmp/initsid.ora' from spfile;
 Shutdown the instance
 set the following parameters in the pfile /tmp/initsid.ora
 undo_management = manual
 event = '10513 trace name context forever, level 2'
 SQLstartup restrict pfile='/tmp/initsid.ora'
 SQL>select tablespace_name, status, segment_name from dba_rollback_segs where status != 'OFFLINE';
This is critical - we are looking for all undo segments to be offline - System will always be online.
If any are 'PARTLY AVAILABLE' or 'NEEDS RECOVERY' - Please open an issue with Oracle Support or update the current SR. There are many options from this moment and Oracle Support Analyst can offer different solutions for the bad undo segments.
If all offline then continue to the next step
 Create new undo tablespace - example
SQL>create undo tablespace <new undo tablespace> datafile <datafile> size 2000M;
 Drop old undo tablespace
SQL>drop tablespace <old undo tablespace> including contents and datafiles;
 SQL>shutdown immediate;
SQL>startup nomount; --> Using your Original spfile
. Modify the spfile with the new undo tablespace name
SQL> Alter system set undo_tablespace = '<new tablespace created in step 6>' scope=spfile;
. SQL>shutdown immediate;
. SQL>startup; --> Using spfile