When you create ad groups you need to include a working link to the app in the app store (either the AppStore or Google Play). The system will use the link to get and update data from the app store.
If you couldn't retrieve additional data from the app store (about icons, ratings, price, number of reviews, etc.), then the ad will only display the data that it was able to get.
Ads will be updated automatically within 24 hours if no other actions are undertaken by the advertiser when information about the app is updated in the app store.