With the end of the Preview for the Office Mobile apps on Windows 10, it is necessary to update your Office Mobile apps. The prompt to perform this update is "Time to get your new Office" with an "Upgrade Now" button. Clicking on "Upgrade Now" does not upgrade the app.
To workaround this problem, you need to remove the licensing file and then Update:
Right-click on the Windows 10 Start button and click Run
Select the following text and choose copy or CTRL + C:
Paste the text into the Run box using CTRL+V and click OK to open File Explorer to this location
Right-click on the file named data.lts (or just data) and choose Delete
You can now launch your Mobile app and upgrade it as needed