Find the "Remove" link under "Current Installations" and click it (see screenshot below)
If you manually installed WordPress
You'll know if you manually installed WordPress. It would have required that you uploaded the WordPress files, setup your database, and went through WordPress' installation process.
To remove WordPress that you manually installed, you would need to use FTP (or the file manager) and delete all of the files in your WordPress folder. After doing this, you would then need to delete your MySQL Database.
Be careful though when deleting files. If you created a store within your WordPress site, such as domain.com/store, be sure not to delete the store folder.
Bottom line, if you're deleting files and you're not 100% sure what you're doing, Create a Backup first.
I hope this helps. Let us know if you have any further questions.