You can change the username with the Ultimate membership.
For that, you need to open the Settings and Privacy tab and scroll to the My Username panel. To proceed further click the "Change Username" button, choose your new username, fill in your current password, and click "Save":
Please note that you can change your username once every 90 days. The change is applied immediately. Also, your old username will become forever unavailable to anyone it also means that you won't be able to change it back.
If you wish to use another username but you don't have Ultimate membership you must create a new account with a different email address.