Ora-01555

Hi All,
Please let me know the steps how we can tune undo_retention for ora-01555. I am listing stats below:

 INST_ID BEGIN_TIME            Stolen     Reused      Error        Error     Length TUNED_UNDORETENTION
---------- ---------------- ----------- ---------- ---------- ------------ ---------- -------------------
         3 12/02/2011 09:22        9352      41374          5            6      49806                6005
         3 12/02/2011 09:32        6619      35034          6            1      50413                6036
         3 12/02/2011 09:42        7884      37289          4            4      51018                6094
         3 12/02/2011 09:52        8542      38552          3            4      34399                6212
         3 12/02/2011 10:02        9660      39705          2            3      27839                6185
         3 12/02/2011 10:12        9064      37144          5            4      28140                6215
         3 12/02/2011 10:22        8367      38316          1            2      28745                6287
         3 12/02/2011 10:32        8685      38672          6            3      29351                6300
         3 12/02/2011 10:42        8667      40706          2            5      29957                6315
         3 12/02/2011 10:52        9734      40536          3            3      30562                6257
         3 12/02/2011 11:02        9045      36636          3            6      31169                6233
         3 12/02/2011 11:12        9617      39758          2            9      31774                6223
         3 12/02/2011 11:22        7874      38278          6            5      32377                6202
         3 12/02/2011 11:32        7634      35825          4            2      32981                6119
         3 12/02/2011 11:42        7576      35509          2            2      33587                6007
         3 12/02/2011 11:52        9665      41675          4            4      34192                5952
         3 12/02/2011 12:02       10783      43405          2            7      34796                5941
         3 12/02/2011 12:12       12371      43544          4            4      35402                5900
         3 12/02/2011 12:22       21737      61278          6           13      36007                5751
         3 12/02/2011 12:32       25378      63692          8           16      36613                5577
         3 12/02/2011 12:42       22190      60706          6           12      37219                5461

                            # Unexpired  # Expired   ORA-1555 Out-Of-space  Max Query
   INST_ID BEGIN_TIME            Stolen     Reused      Error        Error     Length TUNED_UNDORETENTION
---------- ---------------- ----------- ---------- ---------- ------------ ---------- -------------------
         3 12/02/2011 12:52       21148      57513          4            6      37824                5270
         3 12/02/2011 13:02       15745      49115          7            6      38431                5246
         3 12/02/2011 13:12       11750      40563          3            4      39036                5324
         3 12/02/2011 13:22       10865      38043          6            4      39643                5391
         3 12/02/2011 13:32       11975      36886          4            4      40248                5553
         3 12/02/2011 13:42        1360       3939          1            0      40553                5508

1728 rows selected.

SQL> select status,count(*) from dba_undo_extents group by status;

STATUS      COUNT(*)
--------- ----------
EXPIRED        17964
UNEXPIRED      90296
ACTIVE        300003


Regards,

LS