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, click on 'Edit My Profile'.
You will be brought to the 'Edit Profile' page, at the top, you will see the option 'Edit Avatar'. Click on that. Then choose a photo from your device's photo album or even take a photo.
After selecting your photo, you will click 'Save' at the bottom right of the 'Edit Profile' page.