I wouldn't worry about it though. I think that pretty much most of ফ্যানপপ would prefer a different ব্যবহারকারী নাম to the one they have now. I don't know that for a fact obviously, but I've come across a lot of peeople who want to change their username.
It's why so many people make new accounts, but I don't advise that আপনি do that. Because you'll only lose everything you've done and have to start over. And tbh, no one's that bothered about your username.
No আপনি can't change it. The only way to make a new ব্যবহারকারী নাম is to make another ফ্যানপপ account. Now আপনি can মোছা the one that আপনি have, but that's something that আপনি probably don't wanna do. If I were আপনি I would.