Open the app
Check the app’s settings for a Clear Cache or Reset option
Delete and reinstall the app
Offload the app in Settings > General > iPhone Storage > [App Name] > Offload App
Reinstall the app after offloading
Clear Safari cache in Settings > Safari > Clear History and Website Data
Restart the iPhone
Update the app from the App Store
Update iOS in Settings > General > Software Update
Contact the app developer if the app has no cache-clearing option