How do I edit the toolbar actions (move/archive/delete/reply/forward etc)

You can edit the toolbar actions at the bottom of the message detail screen. To do so, follow these steps:

Mobile:

1. Tap (and hold) any of the action icons on the toolbar, and the edit screen will toggle
2. Drag any icons down over existing icons to replace those actions
3. When you're done, tap the Done button.

Desktop:

1. Tap the Email menu (located next to the File menu)
2. Tap Settings
3. Tap the Customize Actions tab