To rename a contact, you must be in the Address Book or on the Opportunity Details page of the contact in question.
From the Address Book, search and select the contact you wish to edit.
Then on the right side of the screen, click anywhere in the Contact Details section to bring up the screen for editing.
A screen will appear to enter information for the contact you are editing.
From the Opportunity Details page, click the pencil icon next to the contact you wish to edit under the 'Contact Info' section.
Remember to save when you're done!