I added a contact through Whatsapp, but I gave them the wrong name. Is there anyway to rename the contact?
Yes, rename the contact in your address book and then inside Whatsapp, click settings and hit refresh favorites. It will update your list and take the new name, you might need to restart the app after.
All the contacts saved using whatsapp are ultimately stored in your address book only. You just have to go to the address book and rename the contact. It will automatically be updated in whatsapp when you restart it or you may manually update the list by going to the settings.