It actually depends on what you are trying to do. In most normal routes, you can only view the cab in the direction that you are driving, hence why BVE is called a cab-view simulator. However in other routes, where you are meant to change ends, you right click, then go down to 'Jump To Station', and then scroll down to the tab which says 'Change Ends', and select this. This may be only on routes that have this featur built in.
I hope this helps,
Dan