How to select from variable that is a table name n Postgre >=9.2

i have a variable that is a name of a table. How can i select or update from this using variable in query , for example:

create or replace function pg_temp.testtst ()
returns varchar(255) as 
$$
declare 
r record; t_name name;
begin   
  for r in SELECT tablename FROM pg_tables WHERE schemaname = 'public' limit 100 loop
      t_name = r.tablename; 
      update  t_name set id = 10 where id = 15; 
  end loop; 
  return seq_name;
end;
$$
language plpgsql; 

it shows ERROR: relation "t_name" does not exist