1. If you know how to do it, solve the cube - that's the simplest way. You will end up with a clear understanding of which piece is twisted. Just twist the wrong piece back into place, and load a complete cube state as explained above.
2. If you can't solve the cube, then disassemble and assemble it back together. It's a fairly simple procedure that should take you less than 5 minutes. Please see the following video for guidance: