Undo table space growing up

SQL> show parameters undo_management;

NAME TYPE VALUE

undo_management string AUTO
SQL> show parameters undo_tablespace;

NAME TYPE VALUE

undo_tablespace string APPS_UNDOTS1
SQL> show parameters undo_retention;

NAME TYPE VALUE

undo_retention integer 1800
SQL> show parameters undo_suppress_errors

how can reduce my undo table space?