Please help to modifiy this query for better performance

Question

Please help to rewrite this query for better performance. This is taking long time to execute.
Table t_t_bil_bil_cycle_change contains 1200000 rows and table t_acctnumberTab countains  200000 rows.
I have created index on ACCOUNT_ID
Query is shown below
update rbabu.t_t_bil_bil_cycle_change a
   set account_number =
       ( select distinct b.account_number
         from rbabu.t_acctnumberTab b
         where a.account_id = b.account_id
Table structure  is shown below
SQL> DESC t_acctnumberTab;
Name           Type         Nullable Default Comments
ACCOUNT_ID     NUMBER(10)                            
ACCOUNT_NUMBER VARCHAR2(24)
SQL> DESC t_t_bil_bil_cycle_change;
Name                    Type         Nullable Default Comments
ACCOUNT_ID              NUMBER(10)                            
ACCOUNT_NUMBER          VARCHAR2(24) Y    

Answer 1

Ishan's solution is good. I would avoid updating rows which already have the right value - it's a waste of time.
You should have a UNIQUE or PRIMARY KEY constraint on t_acctnumberTab.account_id
merge rbabu.t_t_bil_bil_cycle_change a
using
      ( select distinct account_number, account_id
  from  rbabu.t_acctnumberTab
      ) t
on    ( a.account_id = b.account_id
       and decode(a.account_number, b.account_number, 0, 1) = 1
when matched then
  update set a.account_number = b.account_number