Now its pretty easy, you need to create you own ruleset file with the change you want. Then, go to options.cfg file (you can find it on, if using win7 C:\Users\<username>\Documents\OpenXcom , and add it and the end of document, after the Xcom1Ruleset. For example:
rulesets:
- Xcom1Ruleset
- Mycoolmod
Always your mod should be after Xcom1Ruleset