How?

Get out your hex editor, find the appropriate part of the file, and get to work reverse engineering that part; then you can make some changes. Things that are numbers are the easiest to change (for example, tweaking the numbers in the Spellmaker to be more balanced would be easier than making changes to the code).

If you want to make code changes, you are going to need a suitable debugger (I believe DOSBox has one that is not enabled in the default build), and I hope you understand x86 assembly language.