How do I restart a mission?
You can restart missions by pausing the game and whilst in the pause menu selecting the Restart Mission option. This will restart the full mission that you are doing at that moment.
You can also quit a mission by selecting the Quit Mission option in the pause menu.
These two options can only be used during a Mission, when not in a mission these options are grayed out.
How do I reload the last autosave?
You can load the last autosave at any moment and time in the game. This will load the last point that the game autosaved your progress.
You do this by going to the pause menu in the game and navigating down the list and selecting Reload last Autosave.