When starting to dust a computer I start with taking a cloth with rubbing alcohol and rubbing off the dust on the case. After the case is done I take the glass off, the way you take the glass off is basically there should be 4 screw shaped things that you unscrew after they are out you should be able to just take the glass off. Once the glass is off  I take a feather duster and go along the bottom of the inside. 

Moving into the parts and how you clean them. For the fans I take q-tips with rubbing alcohol and get in between the fan blade and behind them. If you have a water cooling system then there are two ways to replace the water. The simple explanation is to take the tube and pour it out or make a vacuum and get it out that way. For the motherboard what i have done in the past is take a feather duster and lightly dust it off. This is all I have done in the past so for the cpu you take the cpu cooler off and replace the thermal paste that is basically it.

Thanks for reading this and I hope it helped