Smooth changes in speed:

currentSpeed = Mathf.MoveTowards(
    currentSpeed,
    targetSpeed,
    acceleration * Time.deltaTime
);

Vector movement:

currentMovement =
    Vector3.MoveTowards(
        currentMovement,
        targetMovement,
        acceleration * Time.deltaTime
    );

Lerp is useful for smoothing, but it does not represent constant acceleration.