On your homescreen, tap on the Profile icon in your upper right corner.
Tap on the red Edit Profile button to edit your Display Name or Bio. Tap on the photo icon to change your photo.
You can have your phone's camera take a photo or you can pick another photo from your phone's gallery.
Once you have your photo, you can adjust the sizing with your 2 fingers, as if you're pinching the photo to resize it. Tap on the checkmark if the photo is to your liking.
Tap on the red Save button to save your new photo.