1)Right click on Mount & Blade in steam, go to browse files.
2) Navigate to the BIN folder, then to the 'Win64_Shipping_client" folder.
3) Scroll all the way to the bottom to find the Version.xml
4) Right click and edit with text document app and delete the content inside and add
<Version>
<Singleplayer Value="e1.5.8.264807"/>
<Multiplayer Value="m1.5.8.264807"/>
</Version>