You need to read from the input stream, then copy that to the output stream.
You can do this byte per byte using the read() and write(int) methods, but experience has tought me that using the read(byte[]) and write(byte[], int, int) methods are a lot faster most of the times.
If you're using the byte[] versions, don't forget that read(byte[]) returns the number of bytes actually read. Don't always just go writing the entire array - you'll write garbage at the end of the file.