How to overwrite only part of a file in c++

Use std::fstream.

The simpler std::ofstream would not work. It would truncate your file (unless you use option std::ios_base::app, which is not what you want anyway).

std::fstream s(my_file_path); // use option std::ios_base::binary if necessary
s.seekp(position_of_data_to_overwrite, std::ios_base::beg);
s.write(my_data, size_of_data_to_overwrite);