Any email you receive from Brilliant contains an 'Unsubscribe' link at the bottom of the page. You can opt out of any and all news updates from Brilliant at any point in time.
To opt back in or further customize your notifications, make sure you're signed in and click here.
Step-by-step instructions
Click the profile photo in the upper right corner
Select Account Settings > Email & Notifications
Select and deselect to customize as you please