Last month, HTC started rolling out the Android 8.0 Oreo update for the unlocked variants of the HTC U11 and U11 Life in the U.S. Earlier today, T-Mobile began rolling out that update for the HTC U11 Life, as confirmed by HTC's official Twitter account.
The update should reach all devices in the coming weeks as the rollout process takes a few days to complete. Alternatively, you may manually check for the update by going to Settings > System > System updates > Check for update. As mentioned by the company in a tweet, the update weighs about 1.32GB, so make sure you're connected to a Wi-Fi network before you start the download.
The latest update brings a handful of new features, including better notifications, picture-in-picture allowing users to open two apps simultaneously, improved autofill, and much more.