If you are clicking the links on the Microsoft website to try and get to the update to 8.1, and you are getting a 403 Forbidden Error, then try hitting the following address in your browser:
ms-windows-store:WindowsUpgrade
Just copy and paste that into your browser address bar, hit enter, and you should be taken to the store to get the update.
This worked on my Surface RT and laptop with Windows 8 Pro.
This is the usual way i've seen apps linked to in the store previously, i'm not sure why it isn't the link Microsoft just put directly into the page.
EDIT: Looks like the link has now been updated on the various Microsoft pages to use the above
