MacOS
The topmost account will be set as the default 'From' address' if you have selected 'All Account : Inbox'. You can drag and drop to re-order the account in your macOS device.
iOS
The topmost account will be set as the default 'From' address' if you have selected 'All Account : Inbox'. On iOS, you can change this via Canary > Settings > Account > Edit.
Please note that this is currently not possible on Android devices.