if you see in the top right corner, there are some icons. click on the icon that look like a gear, it will bring you to 'update profile' page. (or you can also access the page through your profile page. click on the 'profile' option to drop down some more menu. choose 'update profile')

on 'update profile' page, scroll down until you find 'display picture' field. paste the link of your chosen pic there. don't forget to save and you're done.