Fix Default App Associations

The default app for a particular file type is the one that opens when you double-click on a file saved in that format (so Word opens for .doc files, for example). The Windows 10 upgrade process can break some of these associations, and the easiest way to get them back is by opening the 'Settings' app from the Start menu and clicking on the 'System' link. Go to the 'Default Apps' page and you can make any necessary changes, as well as resetting file associations back to their 'Microsoft recommended' ones if required.