Here is a link to the install instructions
https://flightsimulator.zendesk.com/hc/en-us/articles/360019657180-How-to-download-World-Update-III-United-Kingdom-Ireland-
and a link to the troubleshooter.
https://flightsimulator.zendesk.com/hc/en-us/articles/360015760660-Download-Installation-issues-Windows-Store-troubleshooting
I hope these are some help!