If you have zigbee OTA enabled on your hub then your device should automatically download and apply the update. In the best conditions the update for a battery powered sensor from Iris or SmartThings takes about an hour but it can take longer if there are multiple happening at once. Also sometimes the download gets interrupted and it will just try again the next day. If any sensors still haven’t updated after a few days it may help to move them closer to the hub until the update completes.