It isn't possible to change a fanpop username, sorry. I guess it'd be confusing for other members if they got used to associating wewe with a particular name, so wewe just need to be careful to pick a name you'll keep liking when wewe join. The only option would be to futa your current account and start a new one - but then you'd lose all your current fans, your gold, medals, the association with your contributions etc.
fanpop doesn't allow jina la mtumiaji changing. I learned that picking a jina la mtumiaji based off one of my current interests may not have been the best idea. I mean, it was at the time, but interest can fade. So, if you're wanting to a start a new account, try picking a name that's not based around your interests.