If your account is less than 3 days old, you can change the username yourself by editing your profile in the “Preferences” tab. There is a pencil icon near the username you can click to start the editing. Be aware that this will break any @-mention references in existing posts, so use with caution.
If your account is older than 3 days, you will need a forum administrator to help you change your username. Send any administrator on the About Page a private message and they will help.
See this thread on Discourse Meta to learn more about username changes.