Press Option 1 on any mission briefing screen to cycle to the first mission of the next sequence of missions. For example, pressing Option 1 on any tutorial mission’s briefing screen will take you to the first mission in the Reconnaissance sequence.
On the mission briefing screen, hold left or right and press Option 2 to select a specific mission. You can do this for any mission except Firebase #2.
Enter the below cheats at the title screen...
Expert Mode - Press Option 1 + B + A
Full Ammo on Every Mission - Press Option 1 + Up + B
Stealth Armor on Every Mission - Press Option 1 + Down + B