its pretty easy to do yourself. youve got a spin on filter to replace, and a drain bolt that will drop about 2 gallons of tranny fluid. I recommend getting transynd. its pretty spendy, like 35-40 dollars a gallon, but synthetic, and when you are fully synthetic with transynd in the tranny, its good for 150K
the entire system holds like 5-6 gallons tho, and takes 2 to three changes to be fully synthetic
www.dieselplace.com is a great resource and a wealth of knowledge for duramax owners
good luck
we do have some mr goodwrenches on here too that might chime in