You can personalize your profile from the Edit Profile screen, reachable from your Profile page.
Your display name is separate from your username and is limited to 30 characters — this is the friendly name other players see, while your username (the one with the @ in front) stays fixed unless you change it separately in Settings. Your bio has a short length limit too, so keep it snappy — it's meant for a quick line about yourself, not a full paragraph.
You can also pick a favorite game to show off, choose up to two info tags to display on your profile (like your gender, play time, country, or online status), and select up to three grid cards to feature. If you've unlocked album pictures from the Mall, you can also set one as your profile banner.
Don't forget to save your changes before leaving the page — unsaved edits won't be kept.