create or alter procedure PRC
as
declare variable A_T2 integer;
declare variable ID_T1 integer;
declare variable i integer;
begin
  i = 0;
  --Сортируем по id
  for select a from t2 order by id into :a_t2 do
  begin
     -- c помощью skip берем id из нужной строки
     select first 1 skip (:i) id from t1 order by id into :id_t1;

     -- Обновляем :-)
     update t1 set t1.a = :a_t2 where t1.id = :id_t1;
     i = i+1;
  end
end