Go to the top right of the header on the site and click on the dropdown menu.
Click on the first option "View Profile"
On the bottom right of the gray cover section click on the camera icon that says "Upload Photo".
Clicking this will open your browser to choose a photo to upload. Choose a photo to upload and click open to upload.
Once uploaded you can reposition the photo in the cover space to your liking.
Once done, click on the Save button and you're all set!