It's possible, but well-hidden.
Any given meetup seems to have two pages. One is the discussion article, and one is the meetup page. For your meetup, these pages are:
http://lesswrong.com/lw/lbr/meetup_sydney_rationality_dojo_approaches/
http://lesswrong.com/meetups/17g
You can edit from the meetup page: there should be a link right below the map. Changes you make will propagate to the discussion article.
The meetup page is the one that gets linked to from http://lesswrong.com/meetups/ , and the discussion article is the one found on your overview page. Where the discussion page says "discussion article for the meetup :", the meetup title is an unmarked link to the meetup page.
Thank you, that fixed the problem! Maybe the link should be larger or placed in a different location. My guess is I didn't parse it because of its proximity to the map where you usually get map-related links.
I've encountered some bugs in creating and editing meetups, numbers 429, 432, and 479 on issue list here. Bug 432 is that when I try to change the time, another copy of the meetup post is created, and the duplicate posts (as well as the original) can't be deleted.
Several times now I've had problems with meetups posting with the wrong times, and then been unable to fix them. Am I missing something on the meetup page that I could go to to correct the meetup, or is this impossible at the moment? Alternatively, is this something a moderator can do, and if so, who should I speak to, to get a meetup fixed?