Microsoft Flight Simulator X: Steam Edition has files and folders located in numerous locations on your computer system. Please note: You will need to ensure you have the option to show hidden folders enabled.
To perform a clean install you need to remove the following content and folders:
• [drive]:\Program Data\Microsoft\FSX
• [drive]:\Users\<User Name>\AppData\Local\Microsoft\FSX
• [drive]:\Users\<User Name>\AppData\Roaming\Microsoft\FSX
• In your Steam client, right click on the game and select Delete Local Content. A pop-up message should appear confirming that you want to proceed. Select Delete.
• Once done go to [drive]:\Program Files (x86)\Steam\SteamApps\common\FSX. Once here delete all of the content inside this folder and then the folder itself.
• Delete the “Microsoft Flight Simulator X Files” folder in “My Documents”, which can be found at [drive]:\Users\[user name]\Documents\Flight Simulator X Files. Again delete all of the content inside this folder followed by the folder itself.
Once you have deleted all of the above you can perform a clean install of Microsoft Flight Simulator X: Steam Edition by selecting 'install' from the Steam client whilst the game is selected.
Note: this will also delete any 3rd party add-ons you have installed on 'Microsoft Flight Simulator X: Steam Edition'.