How to rotate the main body so its like a twisted body

I want to be able to twist the body so the segments are diagonal.
How can I do this?

Thanks

How much do you want to twist?
And, what segments are you talking about and diagonal to what?

I only want to twist it very slightly like 15 degrees, but when I click on the vertices to rotate the top but whilst the bottom part remain stationary (to twist it, the top part does not rotate

Assuming that’s a single body, that is a pretty complex shape to twist. Have you tried selecting all of the top most vertical and horizontal faces of the extrusion and using the move tool to rotate?