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Please see the official X‑Plane Support page for help.
I am not with Laminar Research; just a flight simmer.
Others before you have wanted to roll back to an earlier version for one reason or another.
The official answer(s) and other flight simmer's comments are the same. "If you haven't made a backup copy, then it can't be done"
Unfortunate news. It appears you will have to wait until flyinside performs an update. Have a look at the following link and the answer from Jennifer (jroberts, Laminar Research) at http://questions.x-plane.com/14539/how-to-downgrade-to-11-05-from-11-10r3
This mob may be able to assist as they are into VR and use flyinside. The link is at https://www.youtube.com/user/BambinoBlog/videos
This site is no longer being actively maintained.