Most newer desktops from Dell, open one of two ways, either push a button for release mechanism, then open like a clam -OR- pull on some spring-loaded latch and allow side covers to fall or grab and pull away. Your model does it one of those ways, unless some screw(s) is holding it back but that's would be obvious, I hope. Remove screw(s) and then apply the above latch mechanisms. While that all sounds easy, at times, because of the tightness or dust build-upo, etc., you may have to play with it. You get the idea once it's revealed to you. Also, you can surf the Dell support website and locate your model# and check for replacement of some part. usually, some generic info is provided for disassemby for int. access.
i have looked on the web for 30 mins and i cant find anything about how to open my dell inspiron 560 MT please can someone helpp me its about a year old now and i what'd to clean the fans