What you're wanting to do used to be called 'object-level patching'. Depending on the machine the code was written for, there should be some kind of dis-assembler available which would allow you to decode the executable back into assembler code. N.B., this is NOT a trivial pursuit. Have done this on IBM mainframe system, and it can be either fun or incredibly tedious, depending on one's peculiar bent of mind.

You can do it with DOS's debug program if you know what you really want to change. Tough to change anything but text strings though. The kiddies have been doing this to game disks for years.