If a new version is available, there will be an extra menu named Update (New!) in the Hoiio main menu. Click on the option and you will be prompted to download the latest Hoiio version.

If you do not see "Update (New!)" in the main menu, this means your version is the latest and there is no newer version. You can always check your Hoiio version number by opening Hoiio and click Settings > Options > About.