If you have not done so already, you can download TouchCopy from here.
This Support article details how to use TouchCopy to copy WhatsApp messages from your iPhone to your PC or Mac.
If you have WhatsApp messages on your iPhone that you need to transfer to your PC/Mac, TouchCopy can copy them in the same way that you can copy SMS and iMessages.
Firstly, you will need to open TouchCopy and connect your iPhone. Once TouchCopy has read your device, navigate to the 'Messages' tab (1) on the left hand side of TouchCopy. You can then scroll down the messages to see your different messages, grouped by type. Your WhatsApp messages will be at the bottom of the list.
From there you can then select the message that you wish to copy or print by clicking on the name of the contact with whom the conversation is with (2). Then, you can either select 'Copy Selected Conversations' or 'Copy to PC/Mac' to copy the selected WhatsApp message to your PC/Mac as a HTML file named after the contact with whom you've exchanged messages; Or you can select 'Print' to print out the selected conversation directly from TouchCopy (3).