Its pretty easy all you have to do is go to My Friend Codes, then press edit at the top right corner then it shows a bunch of stuff you ignore and then you go to Animal Crossing: City Folk where it shows your old FC and then you type in your new one then press save changes.
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In order to change your friend code it's pretty simple. Go to your "My Friend Codes" page and there will be an edit button in the corner. Click that as well and you'll be redirected to a list of slots for you to put your Friend Code information. Find the game of your choice, and put the friend code there, or in the case of you, put your new friend code where your old one was.
Signature-------------- don't be alarmed 'cau2e you don't know me yet, 1M ON TH3 PROWL NOW SN1FF1NG 4ROUND TH1S TOWN FOR YOU