In the first section, you'll see your current picture - click on Change Photo underneath the image
Click on "Change photo" again in the pop-up window, and select the image you'd like to use. You can also crop the image if you'd like!
Once you're finished, scroll down to the bottom of the page and click Save changes.
If you're on the app also click on the drop-down arrow next to your current image in the top right corner of the home screen. Next click on Settings, then Change Photo - then there will be prompts directing you through the process.