To make changes to your account information from your computer, click on your profile picture in the top right-hand corner of the fullscreen home page. From there, you’ll have access to your profile information and can change your username.
If you’re on your iOS or Android device, click on the “person” icon on the lower right of your screen. You’ll see an icon that looks like a gear in the top right-hand corner. From there, you can go to your profile settings and change your username.