The application updates automatically (unless you have disabled automatic updates). Your system regularly checks for updates. However, the updating is not immediate. If you have noticed somehow that there is already a newer version of the app, but you have the old one, then make the update manually.
How to manually update the app?
Open the Windows Store app (green tile on your start screen).
Open the Settings charm bar (or press Win+I), select "Updates" and click the "Check for updates" button.
You will see if there is an update available. Then you can install that update.
Open the Settings charm bar (or press Win+I), select "Updates" and click the "Check for updates" button.
You will see if there is an update available. Then you can install that update.