It is automatic through the Microsoft Store. When an update is available, you can manually force the Microsoft Store to download and install it, but otherwise it will be done in the background when the store decides to process application updates. The FastSpring version of MobileSheets is a little different though - that version prompts you when an update is available and you can choose when to install it. That is because that version is distributed with a standalone installer and updates are downloaded from my server.
Mike
Mike