If you're away from the Mech Plant and you want to change your Mech, here's how you do it:

1. Go into the Combat Simulation Chamber and change the Mech to what you want.
2. Start the Sim and as soon as it's started, press escape and exit to DOS.
3. Come back to GBL and you should have the Mech that you chose. This works with all Mechs.

Enter these codes while playing.

Code      Effect
palex     Destroy targeted mech
kaboom    Kill all units in a mission
clark     X-ray vision
kent      Invincible
burr      Disable heat tracking
jumbo     Gives you jump jets
putz      End mission successfully
crillion  Infinite Jump Juice
thundros  Unlimited ammo

Enter your name as FREEBIRTHTOAD and you will get every single mission. There will be gray on the right side of the underground base scene.