To delete WhatsApp backups on an iPhone ,follow these steps ;

1. Open WhatsApp on your iPhone.
2. Go to “ Settings” within WhatsApp .
3. Tap on “Chats” and then select “Chat Backup ”.
4. Tap on “ Back Up Now ”if you haven't backed up recently ( this step ensures you have the latest backup ). 
5. Tap on “ Auto Backup” and turn it off. 
6. Now , go back to the “Chat Backup ” screen and tap on “ Delete Backup”.
7. Confirm the deletion of backup when prompted .