Hi, my name is Julian. Please make sure that you are sign in to the account you originally used to purchase it. Then open the Microsoft Store or Xbox app, where you purchased the game. Go to the Library and look for Microsoft Flight Simulator. Click install and select the edition you own. Once installed, launch the game for it to download the latest content update.
If you happen to have purchased it on Steam, you'll need to reinstall it via the Steam Library instead.
Here is a guide for installing the game https://flightsimulator.zendesk.com/hc/en-us/articles/360016107019-How-to-reinstall-Microsoft-Flight-Simulator.