While on an existing tab, either:
Select a template
Create! Reloading will make sure the boost is installed and stays updated in your current tab.
Navigate to the Extensions Puzzle icon in the top right corner of Arc’s top bar
Select the code icon beside the Boost you’d like to edit to open it in the Boosts editor
To manage/disable boosts, return to the Extensions Puzzle icon in the top right corner of the top bar and select Manage Extensions
Toggle the Boost Off to temporarily disable it, or hit the Remove button to uninstall it entirely.