Updates to Mobile layouts should happen automatically and fairly quickly after saving changes in the AdminUI. A log out and log back in to the iOS app is necessary. If this still does not show the changes, there are other situations that could be in play. This article discusses one such scenario.
If your webserver is outside a firewall the cache syncing process may take longer than normal or not happen automatically. In these cases an IISReset should propagate the new definition to the cache of the webserver the iOS app connects to.