How do Mario and luigi hit the lever to rotate the bridge?

First thing you gotta do is go see Bowser's body where he got shocked, ya'll will run out because the alarm will go off and will shock Bowser again. Go up the stairs out of that area and you'll see two yellow pipes on opposite sides of each other. Go to the yellow pipe that isn't blocked off which is on the right side of the screen. Follow the path up and you'll eventually run into a floating box which you hit and sends bunches on blue shells everywhere on the map. 3 flying Koopa's will show up and tell you that shouldn't have done that then teach you how to use them. After that go back through the yellow pipe and go to the bridge part that you're at. There will be one of those shells you use and it will be in a floating box which you hit and you hit the level longways not from the tip side, so that the bridge will change directions. I'm sure you've got it from here so hope this sorta helped out.