You can’t directly edit the details of a contact in the WhatsApp application. You need to make any changes in your ‘Contacts’ app on OSX and Whatsapp automatically syncs any changes that are made. But sometimes it takes a while to sync across.
To speed it up you can change the contact details on your iPhone which seems to sync faster.
If they are not syncing it could be a privacy setting on you phone. Check out this link: