NOTE: Don't know which of the two cheats work:
++++++++++++++++

Load it and start playing.  Press ESC.  Now, start a new game while pressing
E.  You should now be in the cheat mode.  If that doesn't work, enter SWAMP
THING into the high-score table, and restart a game while pressing E.  Now,
you can change levels by pressing + and -.  Move the joystick diagonal
up/left, press fire, then press E.  You now enter the game editor.  Pressing
ESC returns you to the level selector mode (pressing SPACE from the level
selection mode returns you to the game).  While in the game editor, press
these keys:  P - alter the game's palette (use 1,2,3,4,5 and 6 from keypad),
X - modify the exits (use - and + on keypad to select exit number, then press
S, then use + and - again to set the exit level.  Use cursor to set
position), W - weapons distribution, works like the exits distribution mode.

++++++++++++++++
Start the game as normal, collect the ship and fly to the extreme top 
left of the playing area. Disembark from the ship and drive the buggy
into the corner. Type in STONKER and re-enter the ship. Now pressing D
cycles through the available  weapons (including shield plus weapons), and
pressing C cycles through the levels.  NOTE: when entering a level, it's
always best to station the ship at the top left of the screen again, to
avoid  appearing at an illegal position, or even off-screen. On the
second level repeat the above process, getting the buggy back in the top
left hand corner, and retype STONKER again. It now enables the ship to fly
through walls, and pressing F while hovering over switches allows them to
be collected while in the air.