What OS are you using ? If you are on Linux, you can subscribe to our repositories from http://fishshell.com/files/2.1.0/linux/index.html and update your package.

I'm on OS X and having installed it initially via homebrew, I just did:

   brew update
   brew upgrade fish

I'm not sure how it'll work if you initially installed it via a different method. I believe the standalone package installs into /usr/local, just like homebrew, so maybe installing via homebrew will just update it out of the box?