How to rollback to Android 5.1 is follow the instructions from their supplied link but delete the "checksum.ini" file inside the folder because it is incorrectly configured. By just deleting the "checksum.ini" file, then it is possible to use the flash tool to go back to Android 5.1.

Manual flash link:
viewtopic.php?f=96&t=1027

And one addition to previous guide:

Win10 gives couple of errors if 'rom'.zip is extracted using Windows own filemanager and if tried to install anyways, installation will fail.

But when extracted using 7zip ROM-package extracts without errors and can be installed safely using previous method.

Если после обновления до 6,0 возникли проблемы с зарядкой,попробуй зарядить батарею зарядкой малой мощности 600ma.shurik-57@mail.ru

Does anyone knowbif you need root access to flash the previous version?

AustrailianBlackview wrote:

Does anyone knowbif you need root access to flash the previous version?

No you don't need !