If the app is receiving an update (in most cases this will be announced in the app via a dedicated Article a day or so in advance), make sure you're on the latest version of the app so you can get back to collecting as soon as the maintenance mode has been lifted. 

Note: It may take some time for the new app version to become available in your Google Play or App Store(s). In most cases the update will become available in a few hours after release, however, it may take up to 24 hours to populate in your locale. 

You can check for app updates by checking out the below links: 
Topps Apps: