Replace All Rows in firsttable with Rows from secondtable with Corresponding IDs

I have these two tables:

create table firsttable (id int, speed double precision);
create table secondtable (id int, speed double precision);

insert into firsttable (id) 
values (1), (2);

insert into secondtable (id, speed)
values (1, 12.3), (1, 12.4), (2, 12.43), (2, 11), (2, 13.6), (3,23);

I want to update firsttable's speed column with values from secondtable

required result:

id	speed
1	12.3
1	12.4
2	12.43
2	11
2	13.6

insert into firsttable(speed)
select speed from secondtable
where id in firsttable.id

ERROR:  syntax error at or near "firsttable"
LINE 4: where id in firsttable.id

EDIT

The answer by Tushar gives:

id	speed
1	null
2	null
1	12.3
1	12.4
2	12.43
2	11
2	13.6

EDIT-2

I tried:

INSERT INTO firsttable (id, speed)
SELECT  id,speed 
FROM secondtable 
WHERE EXISTS (SELECT id FROM firsttable);

id	speed
1	null
2	null
1	12.3
1	12.4
2	12.43
2	11
2	13.6
3	23

But then the firt 2 rows and the last row should not be in the answer (see required answer).

From the description of the desired behavior, it appears that the title should have been Replace All Rows in Table1 with Rows from Table2 with Corresponding IDs. The following query will do this:

WITH ids AS
       (DELETE FROM firsttable
         WHERE id IN (SELECT id FROM secondtable)
         RETURNING id)
INSERT
  INTO firsttable (id, speed)
SELECT id, speed
  FROM secondtable
  WHERE id IN (SELECT id FROM ids)
  RETURNING *;