Hello @Adrian100,
On the website you can log in and:
click on your profile photo
click on "Edit profile"
click on "Photos and video"
Then choose to take a photo with your webcam or to upload a file from your computer.
If you have the Android app you can log in and:
tap on "Profile"
tap on your profile photo
tap on the pencil icon on the now larger profile photo
tap on the profile photo again
You'll then be given the option to take a photo or select one from your gallery.
The iPhone app may have a similar process.
Steve.