Upgrade Node.js to the latest version on Mac OS

Currently I am using Node.js v0.6.16 on Mac OS X 10.7.4. Now I want to upgrade it to the latest Node.js v0.8.1. But after downloading and installing the latest package file from nodejs.org, I found that system is still using v0.6.16 instead of v0.8.1 when I typed “node -v” in a terminal. Is there any step that I have missed? Or, should I thoroughly uninstall the old version before installing the latest one?