Publish a statement of intent. NBC should make clear that this is not what it expects of its journalists, and that this was an isolated incident. It needs a bit of distance from the issue. If it doesn't do this, it risks losing the confidence of the public, especially those in the military.
Allow people to express their feelings. News channels are used to people debating highly emotive topics on their forums and social media pages, but this is different. Many will regard the stories this man told as disrespectful to those serving in the forces. At the moment, the NBC News Facebook page has no "Posts to Page", which is unusual. However, people always find a way to make their voices heard, and if you click the comments under unrelated pictures, you'll see that people are talking about the issue already.
By allowing people to talk about the issue on Facebook, NBC has a chance to engage in conversation and try to shape the debate. By shutting conversation down, or giving the impression that it's doing so, NBC will only drive people to other platforms - such as Twitter - where it has no control at all.
Accept responsibility. This can be hard. After all, it wasn't NBC itself that told the tall tales. But Brian Williams is an official representative of NBC and a very high profile one. Anything he says and does reflects on the NBC brand as a whole. The organisation needs to acknowledge this.
This situation could easily be an honest mistake made by someone who had told the story so many times over the years that he no longer realised that it had strayed so far from fact. But the important issue here is the affect it has on the public, and how they perceive both Williams, and NBC. To move on, both the brand and the man need to understand, acknowledge and learn from what has happened, and respect the opinion of its audience.