One of the real pains about W10 and its usability is the fact that it (after updates) resets your chosen default apps for browsing, email, music, etc. KB 3135173 - an example. Many reported that apps disappeared, as well.
"Stop Resetting My Apps" will prevent this from happening, and it's really simple to use...just click on what you wish to prevent becoming the default app all over again.
Of course, you can do it manually by editing the registry, but who wants to play with the registry? Well, many WC members are skilled at doing that and feel confident doing it...but many don't. So this app might just save you time. Also, the gHacks review also teaches you how to edit which key in the registry and this can increase your confidence.
Just two thoughts...make a registry backup before editing it and create a system restore point you can return to if the installation of the app gets borked for whatever reason.
You can download the app here: http://www.carifred.com/stop_resetting_my_apps/