Clicking the massive Windows 8.1 link from the Store’s main page will take you to the main page for the update. Here you’ll find some useful information about new features and some screenshots to let you know what you’ll be signing up for. Provided you like what you see, go ahead and click “Download.”
As I mentioned previously, this update is pretty huge. Depending on the speed of your connection you could be waiting a while. Don’t just sit there watching the progress bar slowly move to the right; switch away from the store and occupy yourself. Windows will finish the download and then run the installation quietly in the background. You won’t even need to provide input. It will let you know when it’s time to restart to complete the upgrade.