Android Marshmallow for Samsung Galaxy Grand Alpha may never be released but folks at XDA have churned out a firmware via its cyanogen mod channels. This brings CM13 aka cyanogenmod 13 to the Galaxy Alpha android phone which is Android 6 Marshmallow. Its developer says a lot of hours were put into this project to iron out the bugs but some may still be there. So be careful.
To install this Cyanogen Mod 13 Galaxy Alpha firmware, you will need to have Galaxy Alpha custom recovery onboard to let you install custom third party firwmares on it.
To install the ROM, you will need to install sboot which is a custom bootloader. Alternatively, you can install LL ROM via Samsung update option. Suggested modem version is: G850FXXU2COK6 or newer.
After that, download the ROM and copy it to a USB drive and use a USB OTG cable to flash the ROM. Keep in mind that this ROM does not come with Gapps but you can easily install them otherwise from opengapps.org.