Open the app and check for a built-in cache clear option in its settings
Go to Settings on your iPhone
Tap General
Tap iPhone Storage
Select the app
Tap Offload App to remove the app while keeping its documents and data
Tap Delete App to remove the app and its data completely
Reinstall the app from the App Store
Sign back into the app if needed
Clear Safari cache by going to Settings, tapping Safari, then tapping Clear History and Website Data
Restart your iPhone after clearing the cache