You'd do that using the Thread Tools menu at the top of the thread display page.

However, for reasons not only you are entirely responsible for yourself, in the meantime this thread has mutated into some rather fundamental discussion. I, myself, for instance, am planning to make a rather elaborate post for which I did quite some research today, but didn't find the time to actually write the post yet, so I postponed it. So, unlike what's usually recommended (and you're doing good to ask for that ), it may be a good idea to still leave this particular thread in the unresolved state for some more time. OTOH, someone may find an appropriate title for a new thread to move that fundamental discussion to and starts it.

At any rate, as the one who started the thread, you're the only one who can mark the thread resolved (maybe moderators can do that too, but they usually have more important things to do), so eventually it's up to you to decide. If you decide to leave the thread unresolved for the ongoing discussion, you may want to have a look at the thread from time to time, to see whether the discussion is finished in your opinion and then mark it resolved in this case. Of course that's not an issue at all if you're still interested in the discussion progress and thus further follow the thread anyway.