For subscriptions purchased Desktop:

Login to account.brave.com, scroll down to Brave Search Premium under Your plan subscriptions and click Cancel plan.

For subscriptions purchased on Android: 

Open the Google Play Store app, tap your profile icon, then Payments & Subscriptions. Find the Brave Search Premium subscription listed here and tap Cancel subscription. You may also cancel  Play Store subscriptions via Google's support site.

For subscriptions purchased on iOS: 

Open the Apple App store, tap your profile icon, then tap Subscriptions. Find the Brave Search Premium listed here, then tap Cancel subscription. You may also cancel App Store subscriptions via Apple's support site.