* install cm11 after cm12.1
    * start in recovery mode (power+volume up+home buttons)
    * restore to factory mode (format data and cache partitions)
    * format system partition
    * mount system partion
    * sideload cm-11-20141115-SNAPSHOT-M12-i9100.zip (latest cm11 for s2 I could find) && gapps-kk-20140105-signed.zip (with gapps-kk-20140606-signed.zip there are some problems) (prefer adb method)
    * reboot