You change both in the settings under “Edit profile”.
- You can change your display name at any time.
- You can only change your @username at longer intervals. The old name stays reserved for you for 90 days, so nobody takes it over right away.
- You can change upper and lower case in your username at any time.
Links to your profile with the old name only work while it is reserved. Let your followers know when you change your name.