Currently, I'm using ReadFile() and WriteFile() APIs to write & read from file. are there any API functions to replace/edit text if the data is large enough to be written again? I only heard about SetFilePointer() but i'm not quite sure how to use it to replace the text from a file.
For example, select a string/char from file say, value '0' of Key2.
Key1 = 0
Key2 = 0
and change it to '1'.
Key1 = 0
Key2 = 1
There are lots of APIs for random access to files. They are all almost completely unsuitable for dealing with text files.
Consider if instead of changing 1 digit, you changed from 1 to 2 - i.e. from 1" to "20" you would have to implement not only the "efficient" random access way of doing things, but also the "inefficient" re-write the file way. As you only ever really want one way of doing things, the "inefficient" one is the one to go for.
The functions GetPrivateProfileString and WritePrivateProfileString might be good for this.
For text files, it's the best choice to read them into memory, do your work, and write them back. for example, read the file contents into a std::string, look for Key2 = and replace the rest of the line with the new value. Then save the string again. You can use std::ifstream to read from a file without a single WinAPI call.