By pre purchased, I'm going to assume you mean pre-order. If so, you can just cancel the order or message whoever you bought it off for a refund. To cancel the order, you can probably find it in your 'recent orders' area.
Trust me, GTA V on PC is totally worth it. Keep the PC version. very soon, they will be releasing DLC packs, and you'll be able to get mods and extra cheats etc. things you'll never get with Xbox.
Mind you, there is already DLC for the PC coming soon and talk of major expansions, xbox won't have that.