I've never taken apart a set of Shimano Brake/Shift levers, from what I've heard you mostly need to replace them when they crap out. However, I have found that sometimes, with older ones that you get on older bikes, that they are not worn out but just sort of gummed up with old grease. I've had some luck with blasting all the moving parts inside with PB Blaster.
They can be easily disassembled just like any other. You simply have to watch the order of washers and such...taking pics as you take them apart helps.
Tear down, clean, polish (these will be anodized so they can be simply cleaned well), relube as you reassemble. Easy.