How do I turn the Main Camera 90 Degrees on the X axis

How do I turn the Main Camera 90 Degrees on the X axis without it focusing or orbiting around a gameobject? Because on my Main menu screen when the player hits the button “Sandbox” I want the camera to turn 90 degrees on the x axis so its looking at another set of 3D Text which shows the different Maps to play on. But I know what to do when the player clicks one of the maps.

Camera.mainCamera.transform.Rotate(new Vector3(90, 0, 0));

Camera.mainCamera.transform.eulerAngles = Vector3.Lerp(Camera.mainCamera.transform.eulerAngles, new Vector3(90, 0, 0), Time.deltaTime*speed);

where speed is a variable in your code to control how fast it rotates to that point.