My Xperia Z is stuck at 85%-100% brightness, what should I do?

I bought this phone from another user. He said that after updating this phone to 5.1.1, the brightness was stuck. It's brighter than my future. Man, the brightness isn't so bad, but what happens with my battery is. It drains so much, I charge longer than the time I use it. I use an app called "Twilight" to dim the screen but it stills drains and it heats up. I would love to stay on 5.1.1 but going back to 4.4.4 isn't so bad after all. Also, my phone has random shutdowns, I can only reboot it by connecting to PC. Will someone give a step by step (with Links) procedure regarding flashing 4.4.4? I am not rooted. TIA


P.S. this was my first post.

Download from this link