To upgrade to Windows 8.1
Go to your App Store and on the first screen you will see Windows 8.1 upgrade screen. You must make sure you have latest Windows Updates install. If you don't see Windows 8.1 Upgrade screen in Store, you need to go to Windows Update (WIN+W, type Windows Update) and download and install windows updates.
Click on that and just follow the install instructions.
Download is about 3.6 GB so be prepare for a long wait, depending on your Internet connection.
As I write this blog, I am in the process of upgrading my other machine to Windows 8.1.
Cheers!