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Press Shift+O. At the top of the screen you can switch from portrait mode or landscape mode. Select your desired position.

Then press V (Selection Tool) then Ctrl/Command + A (Select All) to select all the objects.

Move your mouse to any corner of the selected box. Your mouse should change to this

Hold Shift and Click & Drag to rotate the selected object 90 degrees. There are number a ways to rotate objects, you can also right click the selected object > Transform > Rotate then apply what degree you want.

It does not matter if you rotate the canvas first or the objects first. They are independent of each other. To my knowledge there is no "one-click" method to doing both at the same time.