You might Thinking that this device is dead now but it’s not yet.This Rom is AOSP Rom of Android Marshmallow 6.0 not Cm13 that is Cyanogenmod 13 rom.So lets start the guide on How to install Android m Marshmallow 6.0 rom in Nexus S.
#How to install Android Marshmallow 6.0 Rom in Nexus S
2.Now place the Android m 6.0 rom zip file in sd card.
3.Now reboot to Recovery mode and flash Android marshmallow rom in your device.
4.Now Clear cache and Clear Data.(Full Wipe Preferred)
5.Reboot your device Note:it will take around 20-30min in first boot so be patient.Done! you just flash Android marshmallow 6.0 AOSP rom in your Nexus S.
6.Enjoy Android Marshmallow 6.0 on your Nexus S