HTC One User Manual page 224

224 Google Play and other apps
Downloading apps from the web
You can download apps directly from websites.
Apps downloaded from websites can be from unknown sources.
To protect HTC One and your personal data, we strongly
recommend that you only download from websites you trust.
1. Open your web browser, then go to the website where you can
download the app you want.
2. Follow the website's download instructions for the app.
Before you can install the downloaded app, make sure that you've set HTC
One to allow installation of third-party apps in Settings > Security >
Unknown sources.
After installing the app, make sure to go back to Settings >
Security and clear the Unknown sources option. This can help
protect your phone from installing other insecure apps from the
Web.