The best way to wipe it clean is to delete the database, all of the files on the web server and then start over.  You will lose everything.  Is that what you want? Or do you just want to remove all users and content while your pages and layout remain in place?

The best way to setup DNN on a server is to use the "New Install".  It should be as simple as copying the DNN install zip file to your web server, extracting the contents, update the connection string in the web.config and then browse to your site.  Since you already have an existing database, you should delete and recreate the database as well.

There are a few tools on the DNN Store that provide automatic updates but I have never used them. I would be very cautious of these tools and don't recommend this approach.  While the DNN upgrades are much more stable then they used to be, there are still issues that can come up during and after an upgrade.  It's really best to manage your DNN upgrades manually. 

Before you do anything, take a backup of your database and your web site.  It is an extra step but if anything should go wrong you can always roll back if you have a backup.