Switching from Win8 to Win7 will replace Win8 on your HDD so follow the instructions to backup your current system (1xDVD, 1x16GB USB stick).
All the information is available on the forum in a number of excellent posts - perform a search and read all you can find before you do anything! There are a few 'Gotcha's' you will need to watch for but, as I say, all the info you need is here.
Member RevoMaxx is quickly moving up the ranks of Acer Community. He joined us in September and is already making a difference. It's great to see new members like him make a difference only after a few months. Thanks RevoMaxx!