Other than the Stable channel, Sizzy offers two more (potentially unstable) options: Alpha and Beta. You can use the latter two development versions if you wish to take a sneak peek at the upcoming Sizzy features (or if you're feeling adventurous.) The Beta channel updates every week, while Alpha gets a release almost every day.
To switch channels, click the ⚙ Settings icon in the top-right of the window, and click on About. Here, you can choose the update channel that you want.
Once you save your preference, Sizzy will automatically look for updates and you'll see a popup asking for an update.
Once the download is complete, you'll be prompted to restart the app. After restarting, you'll be on the unstable channel.