Well, firstly, you will need to uninstall the existing version of outlook.
Click Start > Click Run > Then type "Control Panel" exactly like that, without the quotes..then, click Enter, or OK.
You will then be displayed with a box like this..
Click, add/remove programs (the one highlighted), find the existing Outlook application, and click remove.
Then just install the new version
Ahh, I have just realised - I am presuming you are using XP, post back if your not..