There’s two ways to do it.
The first is to select you text box with the move tool, and the hover your cursor just outside one of the corners and the cursor will change to a little rotation symbol. You can click and then drag in the direction you want to rotate, and if you hold down the shift key it’ll snap in whole increments so it will be easy to land on 90°.
The other way is to select you text box again with the move tool, and then click on a shape tool icon from the tool palette. That’ll change the palette view to the shape info, and you can then change the angle of the text box in the Angle field.
Hope that helps,