Save your game, and note how many credits you have.
Open up the savegame in an hex editor (the first save game
is named X0.SAV then X1.SAV etc etc) DO NOT try and edit X99.SAV,
it's the save game list.
Open up windows calculator, switch to scientific view. type in
your credits *withouth the comma* ie. if you have 298.35 credits,
you type in 29835 and click Hex.
It will give you the value of 12A, since in hex each number is
represented by 2 digits, simply put a zero at the start so it
makes 012A.

Now search for this value in the savegame. You will find it twice
and you must change it in both places. The first location is near
BC90, although this *can* change...
To reiterate, convert the whole number *without* the comma. change
both places!
Making fast money: