If you encounter issues while trying to use MetaMask on the Brave browser (such as an error saying MetaMask is not installed, even though it is), or you simply need to set MetaMask as your default Ethereum provider:
1. Click on the hamburger (three horizontal lines) icon in the upper right hand corner and choose the "Settings" menu
2. Scroll down to, or using the menu on the left-hand side, choose the "Wallet" section of Settings
2. Change "Ethereum provider for using Dapps" to "MetaMask"
3. Restart the Brave browser