For those of you who want to upgrade your Galaxy S2 GT-i9100 to CM13 or any Android 6.0.1 Marshmallow ROM, you will need to repartition your system partition so you can install it correctly.
Marshmallow ROMs require minimum of 1GB of system partition while the Galaxy S2 defaults to 500MB. To do this, it’s pretty simple to do, you just need to install repartitioning zip file using latest TWRP recovery.
First, make sure you have a custom recovery such as CWM or TWRP recovery.
Second, boot into your current CWM or TWRP recovery and flash the ISO Rec TWRP recovery. Then reboot into recovery.
Third, flash the repartitioning zip file. Make sure you have backed up everything on your phone beforehand as there is a chance you may lose some of your data during repartitioning! Once that’s done, reboot back into recovery.
Fourth, once rebooted into TWRP, you can now do a standard installation of the ROM, see our universal failproof guide on how to install Marshmallow ROMs (or see the video tutorial above).