Cancel ios 8 while downloading

Assuming it is being downloaded over a Wi-Fi connection, do a hard reboot. Hold down the Home and Wake/Sleep buttons at the same time for about 15-20 seconds until the Apple logo appears. Ignore the "Slide to power off" text if it comes up. You will not loose anything. Of course the next time you plug it in to charge it and you are on Wi-FI it will start the download again. You don't have to install it however. It will always require selecting "Install" for the installation to complete.