Earlier this week Microsoft updated the Microsoft Band firmware along with the Windows Phone and Android Microsoft Health apps. Microsoft also updated the iOS version which is available in the iTunes store now.
The update takes the app up to version 1.3.1209 and the release notes say it contains bug fixes. On the Windows Phone version it updated the firmware of Microsoft Band after updating the app so the iPhone or iPad version will probably update the Band after installing as well.
The app is free from the iTunes Store.