There are many online tools available to download youtube videos
here are the steps to use one of them
1) Open respective youtube video in browser
2) Copy the youtube video link from browser
3) Goto the following url
4) copy the url in the box labeled as "To download a video just insert a link "
5) Click "Download" button next to video and not the above one.
6) Enjoy the video on your local computer without an internet connection.