How to physically clean my iMac?

By sultanoflondon Hi all,
I have an iMac 27″, Late 2012 version, which I haven’t cleaned properly since I bought it.
As a result, it has become very dusty, externally, on the stand, near USB ports, the lock mount and especially where the air inlet grille is, behind the screen.
On old tower PCs in the past, I have opened it up and cleaned out the dust which has accumulated in the fans, near the motherboard, etc.
I won’t open up my iMac because it is a much more complicated procedure and not with simple screwdrivers, nuts and bolts.
Also, my iMac has started to show signs of ageing in terms of lag and responsiveness, so I think that cleaning it physically would help to reduce this effect.
How do I clean my computer effectively?
Thank you!

Source: MacRumours