Ah, you bought it and now you can't play? That's bad... Be careful with updating stuff. I can't give you advice on that. Decrypting the NAND is not a problem. While Decrypt9 only supports SysNAND, you may just generate a NAND xorpad and use this to apply it to a EmuNAND backup.Firmware & eShop Spoof -> eShop -> Game downloaded.
I think I can update EmuNAND by using ReiNAND + System Settings->Firmware Update?
The problem will be that I need to decrypt the NAND, right? Is it possible after updating it? ReiNAND and N3DS only supports EmuNAND up to 9.5...