Firstly, please make sure you are connected to a stable internet connection. For iOS user's and the website, please log out and log back in. This should refresh the app and sync the system. For Android users please make sure you have the latest version of the app installed in the Google Play Store.
For general syncing in the app (learning progress, ignored words/levels), you can pull down on the Course selector screen to refresh/sync your account on iOS. On Android tapping the star 'Pro' tab will sync the app after a few seconds.
For course content changes made on the website, you will also need to log out and log back into the app to force it to sync with the server - note that this removes any downloaded courses in the app, but you can re-download them.
If you still experience issues, please send us a message by clicking "Did not find an answer to your question? Contact us" on the right hand side.