Hi,
you can remove plugin from Appearance/Plugins in your Dashboard. There you can deactivate Contact form 7 plugin and delete it. After that you can download newest version of Contact Form 7 plugin for free and install it.
But, we think it is not necessary to reinstall plugin, because when we test this we see successful message in green box as it should be.
Here you can read more how to set up Contact From 7 plugin:
http://contactform7.com/setting-up-mail/
Kind regards