You'll need to install Nightly, you can't "update" from one channel to another.

Nightly is not a new name or such but rather is one of the names of the four main development channels. Release, Beta, Aurora and the most unstable which is Nightly (currently has 27.0a1)

Since checkins happen pretty much everyday on Nightly and Aurora there are updates each of those days with sometimes the occasional respin.