Smile for the camera!
To upload a profile photo, be sure you're on your Profile. Click on the empty profile photo (+ profile photo) or by clicking the Edit button to edit in the top right.
A drawer will slide out from the right, simply click on Upload Image, browse for the image or take one if you're on mobile.
Don't forget to Save! When you're done uploading, click Save in the top right corner.
To cancel any changes you've made, simply click the left arrow in the top bar. You will be prompted "You have unsaved changes. Are you sure you want to leave without saving?" Just click 'OK' and your changes will not save.