You will get a message on your device saying 'Please Update' to let you know when you need to update the App.
iOS - can click on the App Store symbol and then tap 'Updates' to see the apps that need updating. Select Rainbow Riches Casino from the list and follow the instructions on screen.
Android - look out for update messages or Push Notifications in the App itself and follow the instructions on-screen.
Alternatively, you can check for updates by going to the Play store and selecting the three lines to open the menu and then 'My Apps and Games'. Here it will check if you have any updates available on your downloads.