Ideally you should try storing all your data in one large file rather than many small ones. A lot of small files take up more disk space than a single large one with the same content, and a lot of small files look messy.
You could read all of the file into a string, read the value you want to change, do a search&replace of that value and replace it with the new value and write the modified string back into the file. To help you not get the correct value you should create a specific format for storing your data (try writing an XML file, for example).

Is it not possible for you to use the game_save() functions that are already included in Gamestudio?