Please refer to below steps to upgrade app:

IOS Device:
  1. Open App Store from your device
  2. Press Updates
  3. Press UPDATE beside desired app
Android Device:
  1. Open Play Store from your device
  2. Press column sign at top left of screen
  3. Press My apps & games
  4. Click INSTALLED tab
  5. Select desired app
  6. Press UPDATE
Note: If you could not find UPDATE button beside desired app, it means this app is the latest version and you have no need to upgrade