I have downgraded syno version before.
Boot with other linux LiveCD, format the DSM disk partition 1 and 2 (two small partitions, they are syno system partitions, without personal data).
BE CAREFUL DO NOT format partition 3 (the biggest one, it's your data partition)!
After format, reboot from old redpill bootloader, you can do reinstall DSM without loss your data, but your system config will lost.
Backup/clone your disk data before if you can.