I guess you can't. That one is relatively new. You can use another free third party extension installer like:
http://www.projectseven.com/products/ex-man/info.htm
or
https://www.dmxzone.com/go/22670/dmxzone-extension-manager-for-dreamweaver/
The both support uninstall. If you have already installed something you would just have to reinstall with the new extension manager first in order to uninstall it.