WhatApp can only be deleted via the app. Make sure you download and install the WhatsApp app before attempting the steps below. 

Click on the Settings icon on the bottom right hand corner of the screen

You have the option to submit a reason for deleting your WhatsApp account. After making that decision, click Next

Click Delete My Account to finish the process. Note: This action cannot be reversed. Once you delete your account, all data tied to it will be deleted as well.

How to Delete Your WhatsApp Account