He developed Android Marshmallow 6.0 Rom for Xperia Z1 and Z Ultra that can be flash via Fastboot.As we already shared the android Marshmallow 6.0 Gapps which is working with AOSP Rom of Nexus Devices and after testing it doesn’t seems to work on these two device.
We Don’t suggest you to flash these Roms for now because These roms are not stable and not meant for daily use and it doesn’t have Google app like playstore ,Account login not possible etc.
You can Track the development on the XDA developer for both the devices from these links
Xperia Z1 Android 6.0 ROM | Xperia Z Ultra Android 6.0 ROM.
#How to Flash Marshmallow 6.0 in Xperia Z1 and Z Ultra.
Note:Your device bootloader must be unlocked before following below steps.
4.Now Extract the Android M 6.0 Rom files in Mfastboot drivers folder.
fastboot erase system
fastboot flash system system.img
fastboot erase boot
fastboot flash boot boot.img
9.Now you can flash Google Gapps 6.0 for android Marshmallow when it is stable for this device.