Which is to say, if you install the 64-bit Developer version from the Desktop blog page it should auto update to the following versions when available - but Developer is still considered unstable and thus not really intended for use as your primary browser. Though I've used it for years now and only had a couple of times it gave me trouble.
Whenever they decide 64-bit has matured sufficiently they will graduate it to Beta and then Stable, but note that Developer will never auto update to Stable.