update t
   set c = 1
 where rowid in (select rowid
                   from (select rowid 
                           from t
                          order by dbms_random.value)
                  where rownum <= 50);

Connected to:
Oracle8i Enterprise Edition Release 8.1.7.1.0 - Production
With the Partitioning option
JServer Release 8.1.7.1.0 - Production
 
sql>create table t as select * from all_objects;
 
Table created.
 
sql>select count(*) from t;
 
 COUNT(*)
---------
     3915
 
sql>update t   
  2     set subobject_name = 'random update' 
  3   where rowid in 
  4     (select rowid                   
  5        from (select rowid                
  6                from t                    
  7               order by dbms_random.value)
  8       where rownum <= 50);
 
50 rows updated.
 
sql>select count(*) from t where subobject_name is not null;
 
 COUNT(*)
---------
       50