If you have signed in to your CurseForge Account through Twitch, You can change your username and avatar through the Twitch Settings panel, under the Profile Tab (Profile Settings, Edit - Pencil icon).

Please make sure to log out and log back in to your account to save these changes.

Alternatively you can change your CurseForge display name by accessing your CurseForge account settings panel.

You will then need to log out and log back in to CurseForge for the changes to apply.

If you're logged in via Google or Discord, it is not possible to change your Username or Profile Picture yet, but you can edit your display name.