The updates are fairly minor, and most are at the front.
The headlights are the most obvious change. This luxury package C-Class, evident by the hood ornament and badge-less grille, shows lights that no longer feature the line of individual LEDs at the top. They've been supplanted by a continuous light bar, which now runs along the top edge of the housing. There's also now one large projector in the unit, with two vertical features next to it. Aside from the lights, the lower grilles are covered up, indicating some changes to the design of those features, but they're not really visible through the camouflage.
At the back, only the taillights show any significant changes. The ovular reverse and turn signal lenses have been eliminated in favor of a slender white stripe at the base of the lights. It's a safe bet they're illuminated with LEDs and do a good job of modernizing the design. The larger red portion hasn't been left out of the facelift fun, either. It now has little reflector or LED stripes running around the edges.
Considering this is a pretty minor update that can be implemented quickly, we'd expect to see them show up on 2018 C-Class models. The cars will probably appear on dealer lots in the next few months.