There are a lot of corrections needed in your code.

• You are opening FILE2 in read mode, you should be opening in the read+write mode.

To begin with, you need to know how to open a file in read+write mode:
Here's the link: sysopen

Then, you need to know how to operate upon a file in read+write mode.

Hope the above information is useful to getting started with what you need to program.