Our Slide shape is bigger than the box platform. But we can change it still now. Because our Line01 is the source shape of the Loft. So any change in Line 01 will change the shape of the loft (Slide). Go to Top viewport and and select the bottom two vertices and move them up.
Then select the topmost four vertices and move them down.
In case you want to have the sides of the slide smooth or roundish, right-click on any of the selected topmost four vertices and click Smooth. You will see that the Slide shape has been changed immediately.