I tried updating the Xbox app, but it consistently failed. At some point I was directed to sign in, but that failed!
The information presented gave me two options
Go to the Store
Xbox support page (sorry, didn't capture it)

The store didn't help, so I went to the support page and tried to sign-in ... again it failed!
I eventually ended up on the Xbox page and was able to sign-in using my MS account (so much for the one account for all services idea )
My account

After signing in (6 different places with the same MS account), I was able to update the Xbox app.

Now here's the rub - the app doesn't know who I am or it assigns a avatar and id to you. I have never used the id SisterMist55167 anywhere ... ever (Nor wood eye) I might use the wizard avatar, but probably not!

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Games and Cortana are good examples of why MS recommends using a MS account to sign-in to Windows. They're brain dead and don't give you the option to sign-in separately. But that's another story ... MS has to fix a lot of assumptions.