How to change transform.Translate over a certain amount of time

Hi I was wondering how I can change a box’s speed over time. EX: I have a box and it is moving at a speed of 5 on the x axis, I need to speed the box up to 10 after 5 seconds. How do I do this? Thanks !!

var mover : Vector3 = Vector3.zero;
function Update()
{
mover = Vector3(5,0,0);
if(Time.deltaTime <= 5)
{
mover.x += 2* Time.deltaTime;
}
else
{
mover.x += 5
}
move();
}

function move()
{
    Transform.Translate(mover * Time.deltaTime,Space.World)
}

sorry I mislead you. So what happens is that I have a box that drops out of a plane. The plane is moving and every time you press the space bar the box drops from it. The box has a movement downwards, so when it drops it hits the ground. I need to find a way so that when the space bar is pressed after, say 10 seconds of gameplay, the box instead of moving downwards at a speed of five, gets moved downwards at a speed of 10, per say.

Does that make sense kind of?