Summarize this article:

If you don’t want to save your current conversation, you can delete it. While still on the chat window, click the button with three vertical dots on the header bar. Then click “Delete chat” from here. This step is optional, and you don’t have to do this if you want to keep your conversation as part of your chat history. When you’re done with WhatsApp Web, click on the button with three vertical dots on the header bar on the left panel. Click “Log out” from here. You will be logged out and brought back to the main WhatsApp Web page with the QR code. If you want, you can continue chatting with WhatsApp on your phone when you’re stepping away from your computer.

Summary:
Delete a chat. Log out. Use WhatsApp on phone.