Installers for older versions are not publicly available for download, and I would recommend not trusting sources that you do not personally know for an installer on a business application.
Instead, please contact MS Support with a bug report and tell them you need a previous version while they work on a fix. They will usually give you one.
The problem, however, is that 2.36 is not compatible with flows that you have edited in 2.38 or 2.39. So, if you have in fact made any changes to your flow (or opened it and saved it without making any changes) with one of these versions, you will not be able to open it or run it in 2.36.
You will need to rebuild your flow from scratch. So, be ready to do that - I recommend copying the entirety of your flow to text files. You can then at least try to paste it back into a new flow when you have downgraded to v2.36. Some of the actions will not be accepted, but you will at least not need to rebuild everything.
See here for reference on how to copy a flow to text files: https://www.wiseowl.co.uk/blog/s2989/copy-flow.htm
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