To rename a file, use the standard function rename() (declared in <stdio.h>). This function takes two arguments of type const char *, the first of which is the old name and the second one is the new name of the file:
include <stdio.h>
int main()
{
int stat=rename("backup.dat", "backup.old");
if (stat)
printf("rename failed");
}
On success, rename() returns zero; otherwise, it returns an error code indicating various types of errors.