Dust is the silent killer for your computer. It gets into your fan bearings and erodes them so they stop working causing your system to overheat. If your system overheats it could catch on fire any heavy dust build up (like pet hair).
Dust will also act as an insulating blanket on your computer chips and components so they will run hotter than they should. This will keep your system slow and effectively reduce the life of the computer.
Firstly you will need to open the case, and beware that when you do this you may be breaking the warranty rules so read your warranty card before hand.
Next remove all the cables attached, especially the power cable. It is always best practice to remember where the USB cables went in which USB port so that the computer does not need to reconfigure itself once you restart the computer.
Take the computer outside and grab a couple of thin sticks and lock them into the fans to stop the fans from spinning when being cleaned else you could stuff the fan bearings.
Using a compressed air gun, but lightly, remove the dust from the computer. Areas include the CPU fan, the exhaust fans, the mainboard, the areas surrounding the hard drive.
DO NOT use compressed air cans. These carry condensation and could cause harm to your computer chips. Also make sure you are always grounded when doing anything inside the computer case.
Remember a healthy computer is a fast computer.