To rename your Custom Fields you will need to go to your Account Settings. Below are the complete instructions:
Please let me know if this worked for you.
Thanks & regards,
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Have a wonderful weekend!
To edit your custom fields, you now want to click on the My Settings tab within your account.
Scroll down to where it says "Contact Settings" and click on the link "Manage my custom fields"
And that will take you here where you can edit your custom fields
Hope that helps!