How make movement speed constant with FPS?

I am trying to code a movement similar to Flappy bird where when I click the button the player jumps.

However whenever the FPS changes the movement change, I understand this is because I am using update() to move the character.

I was taught that if you multiply a value with Time.deltaTime it wouldn’t change. But alas it seems to persist, and there are times when the time.deltaTime value spikes.

What do I do in this scenario?

The code I am using basically gives the player a constant downward acceleration. This makes the velocity goes to the negative. When I want the player to jump, I give it a positive velocity so it flies upwards. Essentially, Flappybird.

void Update () {
	transform.position = new Vector3(
	transform.position.x,
	transform.position.y + (flightDropVelocity * Time.deltaTime),
	transform.position.z);
	flightDropVelocity = flightDropVelocity + flightDropAcceleration;
}

public void Flight(){
	flightDropVelocity = 250f;
}

You are applying the delta time at the wrong place, this is what you want most likely:

void Update () {
    flightDropVelocity = flightDropVelocity + (flightDropAcceleration * Time.deltaTime);
    transform.position = new Vector3(transform.position.x, transform.position.y + flightDropVelocity, transform.position.z);
}