Change username or display name is not easy and you only can get that change if you are a VIP member (with 50 BTC donation) that you are not or you get acceptance from theymos.

To convince theymos changing your username or display name, you must have very solid reason like something is dangerous for your privacy, dox, etc. He barely accepts request from forum members to do that.

only moderators, Donators, and VIP members can change their names automatically
Theymos only changes username to Anonymous on request
How to destroy your account? Change email to random, password & log out
You can not change your username. If your account is new, create a new account, abandon your current account.
How to edit my username?