Hi,
I don't know if you tried that already, but this is what i would do :
1. In Steam, right click on x-plane, properties, and untick beta
2. Write down where x-plane is installed. You can find the path this way :
In Steam, right click on x-plane, browse files (or something like that)
3. From Steam, uninstall X-plane
4. From your file explorer, delete the entire X- plane 11 folder (thanks to step 2)
4. Install X-plane again and this time, it should be the latest stable version.