Well, that depends on your computer, really. Is it a laptop or a desktop? Do you have wireless internet or wired internet?
If it is wired, one easy way is simply to pull the plug from your computer. If you have wireless and use a laptop, most laptops have a button that will disconnect it from the internet.
Another way is to cut the power from your router, but if you router is not next to you that is not really practical unless you have someone to help you or you'll have to run back and forth. ;-)
Also remember the following about the trick:
1. It is differenct from computer to computer how long you'll have to allow it to search before disconnecting. I personally waited 3 seconds but some say they waited 7 and so on. Make different attempts.
2. There are 3 updates that together will give you 7.8. You have to do it with each update - one at a time.