In order to change the currently active camera, select the one you want to be active and press Ctrl0. This will also change your view to the new camera.
To change cameras mid-animation, you need to use markers. A marker can be added by hovering with your mouse over the timeline and pressing M. Next you need to bind the second camera change with that marker. You can do it by selecting the camera and pressing CtrlB.
NB: It is important to know that markers behave a bit like keyframes, so it is important to add one at the beginning of your animation and bind it to your first camera.