You can cancel your Brave VPN subscription at any time. Please use the following steps relevant to the platform you purchased it on:

For subscriptions purchased Desktop:

Login to account.brave.com, scroll down to Brave Firewall + VPN 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 VPN 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 VPN subscription listed here, then tap Cancel subscription. You may also cancel App Store subscriptions via Apple's support site.