If you use Windows XP or below you can go to Start > Control Panel > Add or Remove Programs. In the Currently installed programs box, click on the Google Chrome program and then click Remove.
If you have Windows Vista or above you need to go to Start > Control Panel > Programs > Programs and Features. Select the Google Chrome application and click on Uninstall.
After that, if you want, you can download and install the latest version from the official web page.