Sprint is rolling out the Lollipop update for the HTC One M7 right now, but in case you are disconnected from the Sprint network, don't have access to WiFi, or simply hate waiting, there's an alternative. HTC has posted a full RUU that can flash Lollipop to your phone. It's a bit of a hassle, but you can do it right now.
After downloading the 1.5GB file to your computer, you need to make sure your device is detected when it's plugged in. The fastest way to do that is to install the HTC Sync Manager, which comes with drivers. Then simply launch the file and it should pull up an installation wizard.
Keep in mind this is completely re-flashing the phone, so everything will be deleted. Make sure you have your files backed up.