HTC released the Android 8.0 Oreo update for its midrange U11 life on T-Mobile in December, but if somehow you haven't received the OTA yet and you'd like to flash it manually, you can do so now that the RUU file is available.
It's starting to take quite a while for HTC to publish RUU files after the OTA begins rolling: the unlocked U11 life's took a little less than 2 weeks and now the T-Mobile one has taken even more. But it's been a holiday period and at least HTC does make these files available so that you can flash the update even if you haven't seen the OTA notification just yet. Not all OEMs do that.
Anyway, if you'd like to apply the update, grab the 1.9GB RUU file from the download link below and follow the instructions on the page to get it working. It will wipe your phone though so make sure everything is backed up.
Source : http://www.androidpolice.com/2018/01/05/htc-posts-ruu-file-t-mobile-u11-life-oreo-update/