Just as it did for the Sprint M7, HTC has made a full 1.5GB RUU of Android 5.0 available for the T-Mobile One M8. The OTA is still rolling out, but if you just can't be bothered to wait, or there's some problem getting the OTA to appear on your device, this is the answer. It's not without drawbacks, though.
You'll need to have access to a computer with the proper drivers for your phone in order to use the RUU. Installing HTC Sync will usually make the computer see your device, though. The RUU comes with an installer that should handle the entire flash for you.
Be aware the manual RUU method will completely wipe the phone, so make sure you've got everything backed up first. Check HTC's site for the full instructions. Now go enjoy Lollipop.