In the app, click on the Profile Picture (if there is one) or on the circle at the top of the hamburger menu.
On the profile picture should be a small camera - click on the profile picture to take a replacement photo with your mobile device's camera or upload a photo on your device.
In the Web App, you can change your own Profile picture by clicking on your current profile picture, bringing up your profile page.
Hover the mouse over the current profile picture (or generic circle), then a pencil will appear. Click on the pencil icon to upload a new picture.