Behind the scenes, Ren'Py uses Python's pickle system for serialization, meaning the save data includes basic data types like booleans, integers, strings, and floats, as well as more complex structures and custom class instances. Each save file is actually a ZIP archive containing several components: extra information, the Ren'Py version number, a JSON metadata file, a screenshot thumbnail (screenshot.png), and a log file containing the pickled game state.
While technically a tool to extract RPA archives, UnRen includes a "Save Editor" feature that is legendary. It allows you to edit your saves via a GUI. Renpy Save Editor Github