There's no official way to do this yet.
- You can upvote the feature request here: https://trello.com/c/MYaUdJOR/508-live-development-w-custom-chrome-install-e-g-canary-or-portable-chrome
- This experimental patch makes the browser path user-customizable: https://github.com/adobe/brackets/pull/6622. It might land in the next month or two (I'll update this answer if so).
- You can make a custom build of brackets-shell with hardcoded changes to launch Canary instead of Chrome. See this forum post for details.