You can edit the toolbar actions at the bottom of the message detail screen. To do so, follow these steps:
- Tap (and hold) any of the action icons on the toolbar, and the edit screen will toggle
- Drag any icons down over existing icons to replace those actions
- When you're done, tap the Done button.
- Tap the Email menu (located next to the File menu)
- Tap Settings
- Tap the Customize Actions tab