After signing in at https://www.brillianttv.com/ on a web browser, click on the user icon in the top right, and choose edit profile.
Click Upload a file underneath the avatar section.
Choose a photo from your computer/device and upload it.
Don't forget to click Save on the bottom of the page to save your changes.
You can also update your profile photo from your mobile app. Choose the icon on the top right of the screen and then settings. Then choose upload a photo and follow the prompts. You will get a message that says "Profile Image Updated!" when successful.