How can I change my username?
From Nikonians Wiki - FAQs, Photo Glossary, Good Photo Locations, Help
Short answer: You can't.
Long answer: Since the username is unique and used as an "index key" in the system, it cannot be changed. If you really want to change it, re-register under the new username you want using another email address, then inform the helpdesk using the contact form about the change. Creating a new username this way means you will lose your profile data, your registration date and the number of postings you have.