Open hood, pull the two long vertical 90°(upside-down L or7) shaped pins at the top of the light out and the 'assembly' comes right out to acess the bulb. Remove wire connector then turn bulb counter clock to remove. Do not touch the glass of new bulb , wear a glove or grip by sides as oils from your skin will shorten bulb life.
The answer, including a good disassembled illustration is at: http://www.justanswer.com/questions/1p6yy-how-do-you-change-a-2008-gmc-yukon-h13-headlight The key is to remove the six bolts that attach the facia top to the hood latch bracket, and partially remove the headlamp to get access. Don't forget to loosen but not completely remove the bottom outboard bolt, accessed by partially removing the inner fender.