How to wipe and reset my Wordpress again?

I think I have screwed Wordpress installation by modifying the contents manually. So I tried to use the one-click installation to reinstall it, but it says it’s already installed. I don’t know what I can do now, I simply want to start everything refresh, no need to preserve the existing contents.

Go back to the One-Click Install section and hit Manage Installed Applications. You can select the WP setup and hit Delete all Files.