To change your avatar (profile picture), either click on your username in the top right corner of the screen or click on the profile picture itself. The latter takes you directly to your profile page, the former opens a drop down menu. Click "profile". 

To the left of your profile page, you'll see your current profile picture and a gray square in its bottom left corner that says "profile picture". Click on it. It's set to "upload photo" by default. Click choose and select a picture from your computer, hit continue. In a next step, you can adjust the size of the picture if you so wish. Click save and you're done.