To update your profile picture:
In the top left corner of the app, tap on the 3 lines icon(hamburger menu).
A window will pop-out from the left hand side, tap on your current profile name, in this case it would be 'Your Name Here*'.
You will be brought to the 'Edit Profile' page, at the top, you will see your current avatar with a pencil icon next to it. Tap on that.
Then choose a photo from your device's photo album, or take a brand new photo.
After selecting your photo, you will tap 'Save' at the top right of the 'Edit Profile' page.