NetBoot Image

I am assuming you are talking about switching on the item that you are booting to this NetBoot image. There are a couple of ways to select which image you are booting from. You can hold down the option key on the computer you are booting and select it just as you would any other volume. The other way is to select which one you want to boot from using "Startup Disk" in System Preferences. The NetBoot images should show up there and you can select which one you want to boot from and restart. If you have several NetBoot images, they will show up as a list when you click on a selection that will show there called Network Startup