In most cases, you don't have to do anything. If your device is not set up to update automatically, visit your app store and look for the Deckee app. Your device will give you the option to "update", if there is a new version available. If instead, the store invites you to "open" the app, then you are on the latest version.
How do I check what version I am on?
The Deckee app does not yet display its version, but you can find it by following these steps:
Go to your device settings
Look for Storage
Find the Deckee app and open
Look for the version number