There are various ways. If Cybertenko's promo says you can mirror the prop, the first thing I'd look for would be a mirrored option in the library - but I'll assume you did that.

There are utilities you can buy that will mirror any content that includes geometry. Dimension3D's Content Mirror is one - I haven't tried it, but I'd recommend his stuff in general and I'm sure this works as well as everything he makes.

Alternatively you can mirror the item's geometry in a modeller or a free utility such as STOMP. This may involve messing with the textures, for example if they include text or other stuff that can't be mirrored. Figures probably require more attention to detail than props.

If you bought the item from Renderosity, you're allowed to tell us which one it was. That might help. Somebody here might even have it themselves and be able to give first-hand advice. :)