On your profile, there’s a camera button to the top right of your cover image. Click on that, then scroll down a bit. You’ll see a section to drag and drop a cover image or choose one from your computer. Once here, there’s also a tab right under the wall and profile buttons where you can click to change your profile picture. The process to do this is very similar to the cover image. If you need to find a photo to use, either download a personal photo to your computer, or Google search for a photo you’d like to use, click “Save image as…” and save to your computer so you can then upload to your profile.
Let me know if you need any more help!