what would be the best ? and where I can download it ? kerne change is the only thing I have never changed and I want to learn. thank you.
As I see it, there is currently only one kernel for this firmware that can be recommended and that is Advanced Kernel
http://xdaforums.com/sony-xperia-z1...rnel-advanced-14-5-0-242-kernel-v1-0-t3096060
Flash the .zip in your recovery or (if you don't have one ready)
extract it and flash the boot.img either with an app like "Rashr" or with the fastboot tool on your PC
For further questions on flashing custom kernels, troubleshooting or questions about the kernel itself you can refer to the corresponding sections/threads.
Also leave a like for the dev of the kernel if you end up using it