YOU DON'T. Not ever. Internet Explorer (IE) sucks, big time.
You don't know what default means?????
It means the one you use always. The one your computer launches anytime you want to open a web page or link.
Default = a selection made usually automatically or without active consideration due to lack of a viable alternative - from Webster dictionary
For your web browser, use either Firefox Mozilla, Opera or Google Chrome.
http://www.mozilla.org/en-US/firefox/new/
http://www.opera.com/browser/download/
http://www.google.com/chrome
DO NOT USE INTERNET EXPLORER (IE), as it is the worst browser, and it can be easily attacked and infected.
As an added security measure, also install the WOT (Web of Trust) tool to all your browsers. It rates all websites, blocks access and also alerts you when you open an unsafe website, thus preventing most internet attacks or virus infections.
http://www.mywot.com/
EDIT:
Lool, Adina found the only good use for IE..
Use it only to download other browsers.