I had purchased Iconia W3-810 with Office 2013 pre-installed and activated it using the key on a card in the package.
I have recently reset my device and need to activate Office 2013 again but I have lost the activation key.
How can I get it again?
You are going to have to contact Microsoft. It is always a good idea to both scan and save license keys. I have a small fire-resistant lockbox for mine.