If I visit https://help.nextcloud.com/u/USERNAME/preferences/account, I can change my profile picture, but not my username. I also see no way to delete my account.
Hello, administrative people like me can rename your username. Just contact me if you like.
Deleting an account is not possible in a classical way, this forum software has a anonymous function to clear all user data.
I will provide detail link later.
EDIT:
Please see https://meta.discourse.org/t/anonymizing-users-in-discourse/86929
Thank you. How do I contact you? I see no messaging option on your profile.
We are already in a discussion 
Do you like to have some changes regarding your account?
I would like to anonymize my username at a minimum, and to delete all my content and account if possible, please.
Ok, done this right now.