How to zoom in and out in Game mode?

I am learning Unity using Will Goldstone’s book.I want to know how can I zoom in and out in Game mode.Actually I am using first person controller who has to collide with a door.When I play the scene I seem to get lost and am not able to locate door with which I have to collide.Could someone help please?

On your camera, by changing the value of field of view, you can zoom in and zoom out.