WhatsApp stores your messages and chat on it server for 30 days after which it automatically deletes itself. So on the long run you don’t have to worry about all your messages.
But if you want to force things a little, that is if you want to delete the messages yourself from their server without waiting for them, then here is how to do it.
Note: WhatsApp only store messages that has not been delivered or read, only ones that has not been delivered are stored in their server for 30 days.
What this means is that you can delete your chats from their server that has not be delivered to the recipient. Here is how to do this
– Locate the message you want to delete, check whether it has been delivered by looking at the marks under the message.
– If there is only one mark, then it hasn’t been delivered. Just hold it until it highlights and Trash can appears.
– Tap on that Trash can, and it’s gone from Whatsapp server.