file_encrypt("data.dat", "data1.dat", "key");
file_copy("data1.dat", "data.dat", true);
file_delete("data1.dat");
That's how you'd do it with data.dat, or you could make data1.dat be unencrypted saving you from copying an extra file. But your problem is that the files don't overwrite each other. You could also use string_encrypt which would work just as well, and would quite litterily be easier to restore the data, asuming you arn't working with like a sound file. If you are, do file, if not, string does the trick nicely, like this.
file f;
f.open("data.dat", "rb");
string data=string_encrypt(f.read(), "key");
f.close();
f.open("data.dat", "wb");
f.write(data);
f.close();

I wrote that in a hurry so please forgive any compolation errors, however I hope it helps.