Basic and Advance C Question:

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There isn't a good way. If you need to switch back, the best solution is not to have used freopen in the first place. Try using your own explicit output (or input) stream variable, which you can reassign at will, while leaving the original stdout (or stdin) undisturbed. For example, declare a global
FILE *ofp;
and replace all calls to printf( ... ) with fprintf(ofp, ... ). (Obviously, you'll have to check for calls to putchar and puts, too.) Then you can set ofp to stdout or to anything else.
You might wonder if you could skip freopen entirely, and do something like
FILE *savestdout = stdout; stdout = fopen(file, "w"); /* WRONG */

leaving yourself able to restore stdout later by doing
stdout = savestdout; /* WRONG */
but code like this is not likely to work, because stdout (and stdin and stderr) are typically constants which cannot be reassigned (which is why freopen exists in the first place). It may be possible, in a nonportable way, to save away information about a stream before calling freopen to open some file in its place, such that the original stream can later be restored. The most straightforward and reliable way is to manipulate the underlying file descriptors using a system-specific call such as dup or dup2, if available Another is to copy or inspect the contents of the FILE structure, but this is exceedingly nonportable and unreliable.