I would start searching with Google, how to downgrade your firmware to a previous one!
An example is this: http://androidfact.com/how-to-downgrade-android-firmware-and-versions/
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Just keep in mind, any major OS firmware updates also updates the bootloader of the device. (the most times)
So, it's vey well possible that a specific version of an OS, to require a specific version of a bootloader, and bootloaders are not just easy to change.
You must going to do some serious (re)search, and then acquire the file(s), and try to sort out the method to flash it to your phone.
Also to know, this can void your warranty, as it's a hacking method actually.
And lastly - be aware, doing things like erasing or overwriting the bootloader it's an easy way to brick your phone!
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Also a vey good idea to start with, is to ask on forums to find information about what is available for your particular device.