Deleting an existing site should be a very infrequent action.  ALL the data and site configuration will be PERMANENTLY deleted with this action.  There is no way to recover a site once it's deleted.

Once you are absolutely sure you want to delete a site, click the 'DELETE' button on your site:

The DELETE button on your dashboard will take you to a page where you will be prompted to enter your site name:

You must add your site name to confirm deletion.  In the above screenshot, the site name is 'mysite' which is also displayed right underneath the 'Delete Site' title.

When you are ready to continue, press 'DELETE SITE'.  Once the site is deleted, you will be automatically returned to the dashboard.

NOTE: Deleting your site does not cancel your plan.  Your plan is still in effect and you could create an entirely new site as you did previously.