If you are using a mobile device, click on the three lines/menu bar at the top left hand corner.
Then click on your profile name, should be at the top, and this will take you to your account details.
Scroll down and you should see the section for avatar, click on the image/circle to change, and then you will be able to upload a picture then.
If you are using a computer, it will most probably be similar to the above. Get to account settings, then account details, then scroll down to the avatar section.