iOS apps are updated through the App Store. If you have automatic updates enabled, the app will update automatically. Otherwise, you can update the app manually by opening the App Store, tapping on your profile picture in the top right corner, and scrolling down to the list of apps that have updates available. If you don't see the app in the list, it means there are no updates available.
Android apps are updated through the Google Play Store. If you have automatic updates enabled, the app will update automatically. Otherwise, you can update the app manually by opening the Google Play Store, tapping on the hamburger menu in the top left corner, and tapping on "My apps & games". If you don't see the app in the list, it means there are no updates available.