Both updates fix a security vulnerability.
To download the update to your iPhone or iPad, go to Settings> General> Software Update.
On the Apple Watch, the update can be downloaded now through the companion Watch app on your iPhone by heading to General> Software Update in the My Watch part of the app or on an Apple Watch.
Your watch must be powered at more than 50% and connected to a charger. Like most Apple Watch updates, it can take anywhere from 30 minutes to an hour, so be patient.