While I am not too critical of Google, it appears they dropped the ball on this. It appears that in the beginning, when Google started adding restaurant menus to it's search results, it relied upon using 3rd party sources for the menu data. This makes sense of course, however, Google, at the time and now, cannot know if the data they are presenting is accurate.
Google stated at the time We get all of our menu data from a partner, similarly to how we show other types of answers, like weather. without citing what sites they are getting the data from. Google has a history of secrecy that does not always serve them well. This is one case. Some have speculated that this data came from allmenus.com and viewmenu.com. It is possible that today there are other sources.
There have been some rumbles about how Google has not added the menus to their knowledge graph, however, I believe this is only half true. I am sure that Google was always using the knowledge graph, however, not populating it directly from the restaurant sites, but from niche aggregator sites. All well and good assuming these aggregator sites are up to date.
It is possible, and rightly it should be so, that Google now takes this data directly from restaurant sites or perhaps sometimes does assuming that the data is not available somewhere else. There is no information on this.
If this is so, if Google takes the data from the restaurant site directly, then there is only one course of action you have. Use Fetch and Render to fetch the menu pages and Submit to Index using Googles Search Console. Keep in mind that this step is not immediate, however, the regular googlebot will visit your pages in 1-2 days and make the update official. This does not directly update the knowledge graph of course. However, it may have an influence. Who knows? Otherwise, you would have to wait for Google to get it's act together.
If Google is still using aggregator sites for the source of data, then you can simply search for your restaurant name and something false or out of date in your menu in an attempt to find the source or sources. It may be possible to update your menu on these sites. Even then, if you update your menu data on these sites quickly, you are dependent upon Google to update their knowledge graph. This does not always happen quickly. Sheesh. Damned if you do and damned if you don't.
While I like the feature, the data itself is rarely static and should be updated relatively quickly where ever it exists. Google is in the best position to ensure that the data they present is as up to date as is reasonable providing they are willing to take on the task properly.
<rant>In googles zeal to expand it's offerings and algorithms, complexity, stove piping, and other issues have made quite a few Google fails of late. Google has a long list of fails throughout their history. So what else is new? I just answered another question tonight with another colossal Google fail. This is clearly a weakness in what is an otherwise outstanding feature that can really benefit their user pool and websites alike.</rant>
