Help: how to update to Windows 8.1

Hi,

I've just had my alienware x51 and before starting using it I would like to update W8 to W8.1. The problem is that the link for the update doesn't appear in the Windows store. Why?

I've tried everything Microsoft suggest: all W8 updated are installed, I've reset the store cache,... but the link doesn't pop up. I'm starting to doubt that the copy of W8 that comes with my x51 is not eligible to be updated. Anybody can help?

Thank you in advance.