Delete Old whatsapp Message

You could follow up below steps:
1st - Run WhatsApp on your iPhone and tap any contact or group conversation that includes the messages you want to remove in chats.
2nd - Press and hold on a certain message that you want to delete. Then you will get more options to deal with the selected message, including "Delete", "Copy" and "Forward".
3rd - Tap "Delete", then you will be allowed to tick one or more the messages that you want to delete.
4th - Finally, tap the Trash icon below to remove the single or multiple messages from your iPhone.

1. Open WhatsApp and go to the chat containing the message you wish to delete.
2. Tap and hold the message. Optionally, tap more messages to delete multiple messages at once.
3. Tap Delete at the top of the screen > Delete for everyone.

If you want to delete WhatsApp Account on Phone,you could follow up below steps:

1st - Launch WhatsApp on your Phone and come to "Setting" within the app.

2nd - Tap "Account" > "Delete My Account" to enter another deleting operation section.

3rd - Enter your phone number with full international format: + [Country Code][Full Phone Number] to ensure correct deleting account. Then tap "Delete My Account" again to confirm the deleting process.