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Make sure X Rebirth is NOT running.
Open the Windows Explorer and enter "%USERPROFILE%\Documents\Egosoft\X4" (without the quotes).
You should end up in the X4 user folder here. If you are running the game via Steam, you'd see a subfolder consisting of only numbers (which is your SteamID). In this case, switch into this folder (for example this might result in: C:\Users\[WindowsUserName]\Documents\Egosoft\X4\[SteamID]).
Inside the folder you'd find a config.xml file. Make a backup of the file (in case you need to restore it at some later pointer) and replace the file with this config.xml.
Start X4.