You can only delete messages for everyone for up to seven minutes after sending. Once seven minutes have passed, there is no way to delete messages for everyone.
To delete messages for everyone:
● Open WhatsApp and go to the chat containing the message you wish to delete.
● Tap and hold the message > choose Delete from the menu. Optionally, select more messages to delete multiple messages at once.
● Tap Delete [Trash icon] > Delete for Everyone.
WhatsApp notes that for messages to be successfully deleted for everyone, both sender and recipient must be using the latest version of WhatsApp for Android, iPhone or Windows Phone. It's possible that a recipient will see your message before it's deletion and WhatsApp will NOT inform you if deleting for everyone fails. Click here for more details.
You can download WhatsApp Messenger from the App Store for free.