Your CurseForge account is connected to Twitch, and so you can change your username and avatar through the Twitch Settings panel, under the Profile Tab (Profile Settings, Edit - Pencil icon).
Alternatively you can change your CurseForge display name by accessing your CurseForge 'Profile 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.