How to start modification with big projects
Use Source Control before you touch anything!
There's a book for you: Working Effectively with Legacy Code
It's not about tools, but about various approaches, processes and techniques you can use to better understand and make changes to the code. It is even written from a mostly C++ perspective.
- First study the existing interface well.
- Write tests if they are absent, or expand already written ones.
- Modify the source code.
- Run tests to check if the modification somehow breaks the older behaviour.