I can't tell you exactly how to do it. Mine was supposed to come with a black grille bar from the factory, but Chevy shipped it out with a chrome grille, badges, and lug nuts to get it out the door. About a month later those items came in and were installed by the dealership. I asked for the original parts back and they had completely broken the grille on both sides and slightly scuffed the bottom of my daytime running lights and chipped the paint under the grille on one side. It looks to me like they just pried it out. If I was going to do it myself I would personally pull the grille and bumper assembly off and disengage the clamps from the back. There is a video here for an older pre-refreshed model:
I image it is still pretty much the same. It is definitely worth the upgrade though. Honestly the price doesn't make sense to build it with a black grille, I didn't notice until after I had the vehicle tho. You can buy the grille bar from Chevy accessories for $245 and easily get 10-20% off with coupon codes as well as apply the rewards that you get for buying the Blazer to get it for about $100. But to add it to a build it costs $500 and you are not left with the chrome one as a spare. I will upload pictures of mine to show you the dramatic difference it makes.