Google for Working Effectively With Legacy Code book. Either buy it, which I highly recommend or at least read chapter which is free @ WorkingEffectivelyWithLegacyCode.pdf

1. Identify change points
2. Find an inflection point
3. Cover the inflection point
  a. Break external dependencies
  b. Break internal dependencies
  c. Write tests
4. Make changes
5. Refactor the covered code.