alternative of update function that is not frame dependent ?

enemy launches missiles that chases player,

usually 10, but can launch 100 rockets at ones

    Vector3 PlayerPos = GameObject.FindWithTag("Player").transform.position; // find player position
    rb.AddForce((PlayerPos - transform.position).normalized * 0.4f); // add force in direction of player

this is how those missiles chase player
but the problem is, when i ran this game on high end phone, those rockets were noticeably faster than on a low end device
than i realised update is called every frame so obviously high end device will add more force to missiles than low end device.

how do i get around this ? how do i give them same acceleration in both devices ?

FixedUpdate() is the function independent to frame rate. But adjusting the force with Time.deltaTime is a better option.

You can multiply the force you are adding per frame by Time.deltaTime.
Unity - Scripting API: Time.deltaTime

This will make the added force independent of the framerate.