If you have an iPhone on iOS 8.3 or higher I don't believe you can anymore (believe Apple blocked access to app files). If you are on iOS 8.3 or higher you can always delete the app, restart your phone, then reinstall the app but thats sort of a pain.
If you are on iOS 8.2 or below you can simply use an app like battery doctor or a computer program like PhoneClean to delete the junk files on your phone taking up space.