Update app for Android
If your dictionary has the latest update available, you can easily apply it when you go to The Google Play Store and tap on your profile icon at the top right corner of the screen, then tap on "My apps and games" and go to "Updates". The apps that will have the latest updated version, will be present on the list and you would just have to tap "update", or "Update all" so you applies to all of your apps.
Update app for iOS
If your dictionary has the latest update available, you can easily apply it when you go to the App Store. At the bottom of the screen you will see an "Updates" icon, which most of the time has a red dot on top of it. Tap on the icon and choose the app that can be updated.You can also click on the "Update all" button.