If you use Mozilla Firefox, then complete following steps to get download option in YouTube website,
- Browse https://addons.mozilla.org/en-US/firefox/addon/easy-youtube-video-downl-10137/
- Click "Add to FireFox"
- A window will open. Click "Install"
- Restart your Mozilla Firefox to after completing installation.
Besides there are some online websites where you can download YouTube videos directly. I think this is the easiest way.Those websites are,