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.
Our friend and comrade IronFly has reached his 10,000th post to our little Community. During the last 4 years he has helped countless Acer customers providing help, advice and answers. The Acer Community team feels honored to count you as our friend. Thanks IronFly!