Marvin will automatically check for new updates when you start up the app.
When there is a new update available it starts to download automatically:
If your app is open you will see a notification bar at the bottom when a new update is available. Clicking it will start the download.
Once the download is done, click the notification to restart and install.
Marvin will now run on the newest version.
You can check which version you are running by going to your "account" page.