file modification in linux
 

Hi

Can anyone please tell me how linux saves a file(one that was already saved and is changed now). Does it simply overwrite the entire file or only overwrite the part that was changed.

That depends on how the file was opened. The open() system call depends accepts flags that determine how the file will be handled. Have a look at the open() manual page to see what flags are available:

man 2 open

The developer of an application could have decided to append to a file, change the file, or completely rewrite a file from a buffer.

Thanks for the suggestion. But my question was what the linux kernel does when asked to save a file( for eg: a text document which was modified )

See File handling in the Linux kernel for a detailed complete description. According to the author you will need the following to understand:
Hope that helps.